X-Git-Url: http://wien.tomnetworks.com/gitweb/?a=blobdiff_plain;f=mcs%2Fclass%2FSystem.Core%2FSystem.Linq%2FEnumerable.cs;h=d2c98d0a1954f16a877a18c2c856d759747d2563;hb=cc2c56a916645e45991c43f86409966218a6a177;hp=96e69e57424ec3075c8fb413f867e0d5f5f597e0;hpb=11e8c41da5245c5f42cdd5e94c808d650e6fc81a;p=mono.git

diff --git a/mcs/class/System.Core/System.Linq/Enumerable.cs b/mcs/class/System.Core/System.Linq/Enumerable.cs
index 96e69e57424..d2c98d0a195 100644
--- a/mcs/class/System.Core/System.Linq/Enumerable.cs
+++ b/mcs/class/System.Core/System.Linq/Enumerable.cs
@@ -1,2850 +1,2349 @@
-// Permission is hereby granted, free of charge, to any person obtaining
-// a copy of this software and associated documentation files (the
-// "Software"), to deal in the Software without restriction, including
-// without limitation the rights to use, copy, modify, merge, publish,
-// distribute, sublicense, and/or sell copies of the Software, and to
-// permit persons to whom the Software is furnished to do so, subject to
-// the following conditions:
-// 
-// The above copyright notice and this permission notice shall be
-// included in all copies or substantial portions of the Software.
-// 
-// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
-// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
-// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
-// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
-// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
-// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
-//
-// Authors:
-//        Marek Safar (marek.safar@gmail.com)
-//        Antonello Provenzano  <antonello@deveel.com>
-//        Alejandro Serrano "Serras" (trupill@yahoo.es)
-//
-
-using System;
-using System.Collections;
-using System.Collections.Generic;
-using System.Collections.ObjectModel;
-
-namespace System.Linq
-{
-    public static class Enumerable
-    {
-        #region Aggregate
-        public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, TSource> func)
-        {
-            if (source == null || func == null)
-                throw new ArgumentNullException();
-
-            int counter = 0;
-            TSource folded = default(TSource);
-
-            foreach (TSource element in source)
-            {
-                if (counter == 0)
-                    folded = element;
-                else
-                    folded = func(folded, element);
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return folded;
-        }
-
-
-        public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source,
-            TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func)
-        {
-            if (source == null || func == null)
-                throw new ArgumentNullException();
-
-            TAccumulate folded = seed;
-            foreach (TSource element in source)
-                folded = func(folded, element);
-            return folded;
-        }
-
-
-        public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector)
-        {
-            if (source == null)
-                throw new ArgumentNullException("source");
-            if (func == null)
-                throw new ArgumentNullException("func");
-            if (resultSelector == null)
-                throw new ArgumentNullException("resultSelector");
-
-            TAccumulate result = seed;
-            foreach (TSource e in source)
-                result = func(result, e);
-            return resultSelector(result);
-        }
-        #endregion
-
-        #region All
-        public static bool All<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-                if (!predicate(element))
-                    return false;
-            return true;
-        }
-        #endregion
-
-        #region Any
-        public static bool Any<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-                return true;
-            return false;
-        }
-
-
-        public static bool Any<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-                if (predicate(element))
-                    return true;
-            return false;
-        }
-        #endregion
-
-        #region AsEnumerable
-        public static IEnumerable<TSource> AsEnumerable<TSource>(this IEnumerable<TSource> source)
-        {
-            return source;
-        }
-        #endregion
-
-        #region Average
-        public static double Average(this IEnumerable<int> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            long sum = 0;
-            long counter = 0;
-            foreach (int element in source)
-            {
-                sum += element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return (double)sum / (double)counter;
-        }
-
-
-        public static double? Average(this IEnumerable<int?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long sum = 0;
-            long counter = 0;
-            foreach (int? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (double?)sum / (double?)counter);
-        }
-
-
-        public static double Average(this IEnumerable<long> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            long sum = 0;
-            long counter = 0;
-            foreach (long element in source)
-            {
-                sum += element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return (double)sum / (double)counter;
-        }
-
-
-        public static double? Average(this IEnumerable<long?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long sum = 0;
-            long counter = 0;
-            foreach (long? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (double?)sum / (double?)counter);
-        }
-
-
-        public static double Average(this IEnumerable<double> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            double sum = 0;
-            double counter = 0;
-            foreach (double element in source)
-            {
-                sum += element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return sum / counter;
-        }
-
-
-        public static double? Average(this IEnumerable<double?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            double sum = 0;
-            double counter = 0;
-            foreach (double? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (double?)(sum / counter));
-        }
-
-
-        public static decimal Average(this IEnumerable<decimal> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            decimal sum = 0;
-            decimal counter = 0;
-            foreach (decimal element in source)
-            {
-                sum += element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return sum / counter;
-        }
-
-
-        public static decimal? Average(this IEnumerable<decimal?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            decimal sum = 0;
-            decimal counter = 0;
-            foreach (decimal? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (decimal?)(sum / counter));
-        }
-
-
-        public static double Average<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            long sum = 0;
-            long counter = 0;
-            foreach (TSource item in source)
-            {
-                sum += selector(item);
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return (double)sum / (double)counter;
-        }
-
-
-        public static double? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long sum = 0;
-            long counter = 0;
-            foreach (TSource item in source)
-            {
-                int? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (double?)sum / (double?)counter);
-        }
-
-
-        public static double Average<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            long sum = 0;
-            long counter = 0;
-            foreach (TSource item in source)
-            {
-                sum += selector(item);
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return (double)sum / (double)counter;
-        }
-
-
-        public static double? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long sum = 0;
-            long counter = 0;
-            foreach (TSource item in source)
-            {
-                long? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (double?)sum / (double?)counter);
-        }
-
-
-        public static double Average<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            double sum = 0;
-            double counter = 0;
-            foreach (TSource item in source)
-            {
-                sum += selector(item);
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return sum / counter;
-        }
-
-
-        public static double? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            double sum = 0;
-            double counter = 0;
-            foreach (TSource item in source)
-            {
-                double? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (double?)(sum / counter));
-        }
-
-
-        public static decimal Average<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            decimal sum = 0;
-            decimal counter = 0;
-            foreach (TSource item in source)
-            {
-                sum += selector(item);
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return sum / counter;
-        }
-
-
-        public static decimal? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            decimal sum = 0;
-            decimal counter = 0;
-            foreach (TSource item in source)
-            {
-                decimal? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    sum += element.Value;
-                    counter++;
-                }
-            }
-            return (onlyNull ? null : (decimal?)(sum / counter));
-        }
-        #endregion
-
-        #region Cast
-        public static IEnumerable<TSource> Cast<TSource>(this IEnumerable source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            foreach (object element in source)
-                yield return (TSource)element;
-        }
-        #endregion
-
-        #region Concat
-        public static IEnumerable<TSource> Concat<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second)
-        {
-            if (first == null || second == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in first)
-                yield return element;
-            foreach (TSource element in second)
-                yield return element;
-        }
-
-        #endregion
-
-        #region Contains
-
-        public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource value)
-        {
-            if (source is ICollection<TSource>)
-            {
-                ICollection<TSource> collection = (ICollection<TSource>)source;
-                return collection.Contains(value);
-            }
-
-            return Contains<TSource>(source, value, null);
-        }
-
-
-        public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource value, IEqualityComparer<TSource> comparer)
-        {
-            if (source == null)
-                throw new ArgumentNullException("source");
-
-            if (comparer == null)
-                comparer = EqualityComparer<TSource>.Default;
-
-
-            foreach (TSource e in source)
-            {
-                if (comparer.Equals(e, value))
-                    return true;
-            }
-
-            return false;
-        }
-        #endregion
-
-        #region Count
-        public static int Count<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            if (source is ICollection<TSource>)
-                return ((ICollection<TSource>)source).Count;
-            else
-            {
-                int counter = 0;
-                foreach (TSource element in source)
-                    counter++;
-                return counter;
-            }
-        }
-
-
-        public static int Count<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            int counter = 0;
-            foreach (TSource element in source)
-                if (selector(element))
-                    counter++;
-
-            return counter;
-        }
-        #endregion
-
-        #region DefaultIfEmpty
-
-        public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool noYield = true;
-            foreach (TSource item in source)
-            {
-                noYield = false;
-                yield return item;
-            }
-
-            if (noYield)
-                yield return default(TSource);
-        }
-
-
-        public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool noYield = true;
-            foreach (TSource item in source)
-            {
-                noYield = false;
-                yield return item;
-            }
-
-            if (noYield)
-                yield return defaultValue;
-        }
-
-        #endregion
-
-        #region Distinct
-
-        public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source)
-        {
-            return Distinct<TSource>(source, null);
-        }
-
-        public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            if (comparer == null)
-                comparer = EqualityComparer<TSource>.Default;
-
-            List<TSource> items = new List<TSource>();
-            foreach (TSource element in source)
-            {
-                if (!Contains (items, element, comparer))
-                {
-                    items.Add(element);
-                    yield return element;
-                }
-            }
-        }
-        #endregion
-
-        #region ElementAt
-
-        public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, int index)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-            if (index < 0)
-                throw new ArgumentOutOfRangeException();
-
-            if (source is IList<TSource>)
-                return ((IList<TSource>)source)[index];
-            else
-            {
-                int counter = 0;
-                foreach (TSource element in source)
-                {
-                    if (counter == index)
-                        return element;
-                    counter++;
-                }
-                throw new ArgumentOutOfRangeException();
-            }
-        }
-
-        #endregion
-
-        #region ElementAtOrDefault
-
-        public static TSource ElementAtOrDefault<TSource>(this IEnumerable<TSource> source, int index)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-            if (index < 0)
-                return default(TSource);
-
-            if (source is IList<TSource>)
-            {
-                if (((IList<TSource>)source).Count >= index)
-                    return default(TSource);
-                else
-                    return ((IList<TSource>)source)[index];
-            }
-            else
-            {
-                int counter = 0;
-                foreach (TSource element in source)
-                {
-                    if (counter == index)
-                        return element;
-                    counter++;
-                }
-                return default(TSource);
-            }
-        }
-
-        #endregion
-
-        #region Empty
-        public static IEnumerable<TResult> Empty<TResult>()
-        {
-            return new List<TResult>();
-        }
-        #endregion
-
-        #region Except
-
-        public static IEnumerable<T> Except<T>(this IEnumerable<T> first, IEnumerable<T> second)
-        {
-            return Except<T>(first, second, null);
-        }
-
-        public static IEnumerable<TSource> Except<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
-        {
-            if (first == null || second == null)
-                throw new ArgumentNullException();
-
-            if (comparer == null)
-                comparer = EqualityComparer<TSource>.Default;
-
-            List<TSource> items = new List<TSource>(Distinct(first));
-            foreach (TSource element in second)
-            {
-                int index = IndexOf(items, element, comparer);
-                if (index == -1)
-                    items.Add(element);
-                else
-                    items.RemoveAt(index);
-            }
-            foreach (TSource item in items)
-                yield return item;
-        }
-
-        #endregion
-
-        #region First
-
-        public static TSource First<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-                return element;
-
-            throw new InvalidOperationException();
-        }
-
-
-        public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-            {
-                if (predicate(element))
-                    return element;
-            }
-
-            throw new InvalidOperationException();
-        }
-
-        #endregion
-
-        #region FirstOrDefault
-
-        public static T FirstOrDefault<T>(this IEnumerable<T> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            foreach (T element in source)
-                return element;
-
-            return default(T);
-        }
-
-
-        public static T FirstOrDefault<T>(this IEnumerable<T> source, Func<T, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            foreach (T element in source)
-            {
-                if (predicate(element))
-                    return element;
-            }
-
-            return default(T);
-        }
-
-        #endregion
-
-        #region GroupBy
-
-        private static List<T> ContainsGroup<K, T>(
-                Dictionary<K, List<T>> items, K key, IEqualityComparer<K> comparer)
-        {
-            IEqualityComparer<K> comparerInUse = (comparer ?? EqualityComparer<K>.Default);
-            foreach (KeyValuePair<K, List<T>> value in items)
-            {
-                if (comparerInUse.Equals(value.Key, key))
-                    return value.Value;
-            }
-            return null;
-        }
-
-
-        public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, 
-            Func<TSource, TKey> keySelector)
-        {
-            return GroupBy<TSource, TKey>(source, keySelector, null);
-        }
-
-
-        public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source,
-            Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null)
-                throw new ArgumentNullException();
-
-            Dictionary<TKey, List<TSource>> groups = new Dictionary<TKey, List<TSource>>();
-            List<TSource> nullList = new List<TSource>();
-            int counter = 0;
-            int nullCounter = -1;
-
-            foreach (TSource element in source)
-            {
-                TKey key = keySelector(element);
-                if (key == null)
-                {
-                    nullList.Add(element);
-                    if (nullCounter == -1)
-                    {
-                        nullCounter = counter;
-                        counter++;
-                    }
-                }
-                else
-                {
-                    List<TSource> group = ContainsGroup<TKey, TSource>(groups, key, comparer);
-                    if (group == null)
-                    {
-                        group = new List<TSource>();
-                        groups.Add(key, group);
-                        counter++;
-                    }
-                    group.Add(element);
-                }
-            }
-
-            counter = 0;
-            foreach (KeyValuePair<TKey, List<TSource>> group in groups)
-            {
-                if (counter == nullCounter)
-                {
-                    Grouping<TKey, TSource> nullGroup = new Grouping<TKey, TSource>(default(TKey), nullList);
-                    yield return nullGroup;
-                    counter++;
-                }
-                Grouping<TKey, TSource> grouping = new Grouping<TKey, TSource>(group.Key, group.Value);
-                yield return grouping;
-                counter++;
-            }
-        }
-
-
-        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source,
-            Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
-        {
-            return GroupBy<TSource, TKey, TElement>(source, keySelector, elementSelector);
-        }
-
-
-        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source,
-            Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null || elementSelector == null)
-                throw new ArgumentNullException();
-
-            Dictionary<TKey, List<TElement>> groups = new Dictionary<TKey, List<TElement>>();
-            List<TElement> nullList = new List<TElement>();
-            int counter = 0;
-            int nullCounter = -1;
-
-            foreach (TSource item in source)
-            {
-                TKey key = keySelector(item);
-                TElement element = elementSelector(item);
-                if (key == null)
-                {
-                    nullList.Add(element);
-                    if (nullCounter == -1)
-                    {
-                        nullCounter = counter;
-                        counter++;
-                    }
-                }
-                else
-                {
-                    List<TElement> group = ContainsGroup<TKey, TElement>(groups, key, comparer);
-                    if (group == null)
-                    {
-                        group = new List<TElement>();
-                        groups.Add(key, group);
-                        counter++;
-                    }
-                    group.Add(element);
-                }
-            }
-
-            counter = 0;
-            foreach (KeyValuePair<TKey, List<TElement>> group in groups)
-            {
-                if (counter == nullCounter)
-                {
-                    Grouping<TKey, TElement> nullGroup = new Grouping<TKey, TElement>(default(TKey), nullList);
-                    yield return nullGroup;
-                    counter++;
-                }
-                Grouping<TKey, TElement> grouping = new Grouping<TKey, TElement>(group.Key, group.Value);
-                yield return grouping;
-                counter++;
-            }
-        }
-
-        #endregion
-
-        # region GroupJoin
-
-        public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer,
-            IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, 
-            Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector)
-        {
-            return GroupJoin(outer, inner, outerKeySelector, innerKeySelector, resultSelector, null);
-        }
-
-        public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer,
-            IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
-            Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector, 
-            IEqualityComparer<TKey> comparer)
-        {
-            if (outer == null || inner == null || outerKeySelector == null ||
-                innerKeySelector == null || resultSelector == null)
-                throw new ArgumentNullException();
-
-            if (comparer == null)
-                comparer = EqualityComparer<TKey>.Default;
-
-            Lookup<TKey, TInner> innerKeys = ToLookup<TInner, TKey>(inner, innerKeySelector, comparer);
-            /*Dictionary<K, List<U>> innerKeys = new Dictionary<K, List<U>> ();
-            foreach (U element in inner)
-            {
-                    K innerKey = innerKeySelector (element);
-                    if (!innerKeys.ContainsKey (innerKey))
-                            innerKeys.Add (innerKey, new List<U> ());
-                    innerKeys[innerKey].Add (element);
-            }*/
-
-            foreach (TOuter element in outer)
-            {
-                TKey outerKey = outerKeySelector(element);
-                if (innerKeys.Contains(outerKey))
-                    yield return resultSelector(element, innerKeys[outerKey]);
-            }
-        }
-
-        #endregion
-
-        #region Intersect
-
-
-        public static IEnumerable<TSource> Intersect<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second)
-        {
-            if (first == null || second == null)
-                throw new ArgumentNullException();
-
-            List<TSource> items = new List<TSource>(Distinct(first));
-            bool[] marked = new bool[items.Count];
-            for (int i = 0; i < marked.Length; i++)
-                marked[i] = false;
-
-            foreach (TSource element in second)
-            {
-                int index = IndexOf(items, element);
-                if (index != -1)
-                    marked[index] = true;
-            }
-            for (int i = 0; i < marked.Length; i++)
-            {
-                if (marked[i])
-                    yield return items[i];
-            }
-        }
-
-        #endregion
-
-        # region Join
-
-        public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer,
-            IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
-            Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector, IEqualityComparer<TKey> comparer)
-        {
-            if (outer == null || inner == null || outerKeySelector == null ||
-                    innerKeySelector == null || resultSelector == null)
-                throw new ArgumentNullException();
-
-            if (comparer == null)
-                comparer = EqualityComparer<TKey>.Default;
-
-            Lookup<TKey, TInner> innerKeys = ToLookup<TInner, TKey>(inner, innerKeySelector, comparer);
-            /*Dictionary<K, List<U>> innerKeys = new Dictionary<K, List<U>> ();
-            foreach (U element in inner)
-            {
-                    K innerKey = innerKeySelector (element);
-                    if (!innerKeys.ContainsKey (innerKey))
-                            innerKeys.Add (innerKey, new List<U> ());
-                    innerKeys[innerKey].Add (element);
-            }*/
-
-            foreach (TOuter element in outer)
-            {
-                TKey outerKey = outerKeySelector(element);
-                if (innerKeys.Contains(outerKey))
-                {
-                    foreach (TInner innerElement in innerKeys[outerKey])
-                        yield return resultSelector(element, innerElement);
-                }
-            }
-        }
-
-        public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer,
-            IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
-            Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector)
-        {
-            return Join<TOuter, TInner, TKey, TResult>(outer, inner, outerKeySelector, innerKeySelector, resultSelector);
-        }
-        # endregion
-
-        #region Last
-
-        public static TSource Last<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool noElements = true;
-            TSource lastElement = default(TSource);
-            foreach (TSource element in source)
-            {
-                if (noElements) noElements = false;
-                lastElement = element;
-            }
-
-            if (!noElements)
-                return lastElement;
-            else
-                throw new InvalidOperationException();
-        }
-
-        public static TSource Last<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            bool noElements = true;
-            TSource lastElement = default(TSource);
-            foreach (TSource element in source)
-            {
-                if (predicate(element))
-                {
-                    if (noElements) noElements = false;
-                    lastElement = element;
-                }
-            }
-
-            if (!noElements)
-                return lastElement;
-            else
-                throw new InvalidOperationException();
-        }
-
-        #endregion
-
-        #region LastOrDefault
-
-        public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            TSource lastElement = default(TSource);
-            foreach (TSource element in source)
-                lastElement = element;
-
-            return lastElement;
-        }
-
-        public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            TSource lastElement = default(TSource);
-            foreach (TSource element in source)
-            {
-                if (predicate(element))
-                    lastElement = element;
-            }
-
-            return lastElement;
-        }
-
-        #endregion
-
-        #region LongCount
-        public static long LongCount<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            long counter = 0;
-            foreach (TSource element in source)
-                counter++;
-            return counter;
-        }
-
-
-        public static long LongCount<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            long counter = 0;
-            foreach (TSource element in source)
-                if (selector(element))
-                    counter++;
-
-            return counter;
-        }
-
-        #endregion
-
-        #region Max
-
-        public static int Max(this IEnumerable<int> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            int maximum = int.MinValue;
-            int counter = 0;
-            foreach (int element in source)
-            {
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static int? Max(this IEnumerable<int?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            int? maximum = int.MinValue;
-            foreach (int? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static long Max(this IEnumerable<long> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            long maximum = long.MinValue;
-            int counter = 0;
-            foreach (long element in source)
-            {
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static long? Max(this IEnumerable<long?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long? maximum = long.MinValue;
-            foreach (long? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static double Max(this IEnumerable<double> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            double maximum = double.MinValue;
-            int counter = 0;
-            foreach (double element in source)
-            {
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static double? Max(this IEnumerable<double?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            double? maximum = double.MinValue;
-            foreach (double? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static decimal Max(this IEnumerable<decimal> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            decimal maximum = decimal.MinValue;
-            int counter = 0;
-            foreach (decimal element in source)
-            {
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static decimal? Max(this IEnumerable<decimal?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            decimal? maximum = decimal.MinValue;
-            foreach (decimal? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static T Max<T>(this IEnumerable<T> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool notAssigned = true;
-            T maximum = default(T);
-            int counter = 0;
-            foreach (T element in source)
-            {
-                if (notAssigned)
-                {
-                    maximum = element;
-                    notAssigned = false;
-                }
-                else
-                {
-                    int comparison;
-                    if (element is IComparable<T>)
-                        comparison = ((IComparable<T>)element).CompareTo(maximum);
-                    else if (element is System.IComparable)
-                        comparison = ((System.IComparable)element).CompareTo(maximum);
-                    else
-                        throw new ArgumentNullException();
-
-                    if (comparison > 0)
-                        maximum = element;
-                }
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static int Max<T>(this IEnumerable<T> source,
-                Func<T, int> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            int maximum = int.MinValue;
-            int counter = 0;
-            foreach (T item in source)
-            {
-                int element = selector(item);
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static int? Max<T>(this IEnumerable<T> source,
-                Func<T, int?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            int? maximum = int.MinValue;
-            foreach (T item in source)
-            {
-                int? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static long Max<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, long> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            long maximum = long.MinValue;
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                long element = selector(item);
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static long? Max<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, long?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long? maximum = long.MinValue;
-            foreach (TSource item in source)
-            {
-                long? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static double Max<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, double> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            double maximum = double.MinValue;
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                double element = selector(item);
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static double? Max<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, double?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            double? maximum = double.MinValue;
-            foreach (TSource item in source)
-            {
-                double? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static decimal Max<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, decimal> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            decimal maximum = decimal.MinValue;
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                decimal element = selector(item);
-                if (element > maximum)
-                    maximum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-
-        public static decimal? Max<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, decimal?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            decimal? maximum = decimal.MinValue;
-            foreach (TSource item in source)
-            {
-                decimal? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element > maximum)
-                        maximum = element;
-                }
-            }
-            return (onlyNull ? null : maximum);
-        }
-
-
-        public static TResult Max<TSource, TResult>(this IEnumerable<TSource> source,
-                Func<TSource, TResult> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool notAssigned = true;
-            TResult maximum = default(TResult);
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                TResult element = selector(item);
-                if (notAssigned)
-                {
-                    maximum = element;
-                    notAssigned = false;
-                }
-                else
-                {
-                    int comparison;
-                    if (element is IComparable<TResult>)
-                        comparison = ((IComparable<TResult>)element).CompareTo(maximum);
-                    else if (element is System.IComparable)
-                        comparison = ((System.IComparable)element).CompareTo(maximum);
-                    else
-                        throw new ArgumentNullException();
-
-                    if (comparison > 0)
-                        maximum = element;
-                }
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return maximum;
-        }
-
-        #endregion
-
-        #region Min
-
-        public static int Min(this IEnumerable<int> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            int minimum = int.MaxValue;
-            int counter = 0;
-            foreach (int element in source)
-            {
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static int? Min(this IEnumerable<int?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            int? minimum = int.MaxValue;
-            foreach (int? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-        public static long Min(this IEnumerable<long> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            long minimum = long.MaxValue;
-            int counter = 0;
-            foreach (long element in source)
-            {
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static long? Min(this IEnumerable<long?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long? minimum = long.MaxValue;
-            foreach (long? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-
-        public static double Min(this IEnumerable<double> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            double minimum = double.MaxValue;
-            int counter = 0;
-            foreach (double element in source)
-            {
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static double? Min(this IEnumerable<double?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            double? minimum = double.MaxValue;
-            foreach (double? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-
-        public static decimal Min(this IEnumerable<decimal> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            decimal minimum = decimal.MaxValue;
-            int counter = 0;
-            foreach (decimal element in source)
-            {
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static decimal? Min(this IEnumerable<decimal?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            decimal? minimum = decimal.MaxValue;
-            foreach (decimal? element in source)
-            {
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-
-        public static TSource Min<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool notAssigned = true;
-            TSource minimum = default(TSource);
-            int counter = 0;
-            foreach (TSource element in source)
-            {
-                if (notAssigned)
-                {
-                    minimum = element;
-                    notAssigned = false;
-                }
-                else
-                {
-                    int comparison;
-                    if (element is IComparable<TSource>)
-                        comparison = ((IComparable<TSource>)element).CompareTo(minimum);
-                    else if (element is System.IComparable)
-                        comparison = ((System.IComparable)element).CompareTo(minimum);
-                    else
-                        throw new ArgumentNullException();
-
-                    if (comparison < 0)
-                        minimum = element;
-                }
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static int Min<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, int> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            int minimum = int.MaxValue;
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                int element = selector(item);
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static int? Min<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, int?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            int? minimum = int.MaxValue;
-            foreach (TSource item in source)
-            {
-                int? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-
-        public static long Min<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, long> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            long minimum = long.MaxValue;
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                long element = selector(item);
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static long? Min<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, long?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            long? minimum = long.MaxValue;
-            foreach (TSource item in source)
-            {
-                long? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-
-        public static double Min<TSource>(this IEnumerable<TSource> source,
-            Func<TSource, double> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            double minimum = double.MaxValue;
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                double element = selector(item);
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static double? Min<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, double?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            double? minimum = double.MaxValue;
-            foreach (TSource item in source)
-            {
-                double? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-
-        public static decimal Min<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, decimal> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            decimal minimum = decimal.MaxValue;
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                decimal element = selector(item);
-                if (element < minimum)
-                    minimum = element;
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-
-        public static decimal? Min<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, decimal?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool onlyNull = true;
-            decimal? minimum = decimal.MaxValue;
-            foreach (TSource item in source)
-            {
-                decimal? element = selector(item);
-                if (element.HasValue)
-                {
-                    onlyNull = false;
-                    if (element < minimum)
-                        minimum = element;
-                }
-            }
-            return (onlyNull ? null : minimum);
-        }
-
-
-        public static TResult Min<TSource, TResult>(this IEnumerable<TSource> source,
-                Func<TSource, TResult> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            bool notAssigned = true;
-            TResult minimum = default(TResult);
-            int counter = 0;
-            foreach (TSource item in source)
-            {
-                TResult element = selector(item);
-                if (notAssigned)
-                {
-                    minimum = element;
-                    notAssigned = false;
-                }
-                else
-                {
-                    int comparison;
-                    if (element is IComparable<TResult>)
-                        comparison = ((IComparable<TResult>)element).CompareTo(minimum);
-                    else if (element is System.IComparable)
-                        comparison = ((System.IComparable)element).CompareTo(minimum);
-                    else
-                        throw new ArgumentNullException();
-
-                    if (comparison < 0)
-                        minimum = element;
-                }
-                counter++;
-            }
-
-            if (counter == 0)
-                throw new InvalidOperationException();
-            else
-                return minimum;
-        }
-
-        #endregion
-
-        #region OfType
-
-        public static IEnumerable<TSource> OfType<TSource>(this IEnumerable source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            foreach (object element in source)
-                if (element is TSource)
-                    yield return (TSource)element;
-        }
-
-        #endregion
-
-        #region OrderBy
-
-        public static OrderedSequence<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source,
-                Func<TSource, TKey> keySelector)
-        {
-            return OrderBy<TSource, TKey>(source, keySelector, null);
-        }
-
-
-        public static OrderedSequence<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source,
-                Func<TSource, TKey> keySelector,
-                IComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null)
-                throw new ArgumentNullException();
-
-            return new InternalOrderedSequence<TSource, TKey>(
-                    source, keySelector, (comparer ?? Comparer<TKey>.Default), false, null);
-        }
-
-        #endregion
-
-        #region OrderByDescending
-
-        public static OrderedSequence<TSource> OrderByDescending<TSource, TKey>(this IEnumerable<TSource> source,
-                Func<TSource, TKey> keySelector)
-        {
-            return OrderByDescending<TSource, TKey>(source, keySelector, null);
-        }
-
-
-        public static OrderedSequence<TSource> OrderByDescending<TSource, TKey>(this IEnumerable<TSource> source,
-                Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null)
-                throw new ArgumentNullException();
-
-            return new InternalOrderedSequence<TSource, TKey>(
-                    source, keySelector, (comparer ?? Comparer<TKey>.Default), true, null);
-        }
-
-        #endregion
-
-        #region Range
-
-        public static IEnumerable<int> Range(int start, int count)
-        {
-            if (count < 0 || (start + count - 1) > int.MaxValue)
-                throw new ArgumentOutOfRangeException();
-
-            for (int i = start; i < (start + count - 1); i++)
-                yield return i;
-        }
-
-        #endregion
-
-        #region Repeat
-
-        public static IEnumerable<TResult> Repeat<TResult>(TResult element, int count)
-        {
-            if (count < 0)
-                throw new ArgumentOutOfRangeException();
-
-            for (int i = 0; i < count; i++)
-                yield return element;
-        }
-
-        #endregion
-
-
-        #region Reverse
-
-        public static IEnumerable<TSource> Reverse<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            List<TSource> list = new List<TSource>(source);
-            list.Reverse();
-            return list;
-        }
-
-        #endregion
-
-        #region Select
-
-        public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source,
-                Func<TSource, TResult> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-                yield return selector(element);
-        }
-
-
-        public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source,
-                Func<TSource, int, TResult> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            int counter = 0;
-            foreach (TSource element in source)
-            {
-                yield return selector(element, counter);
-                counter++;
-            }
-        }
-
-        #endregion
-
-        #region SelectMany
-
-        public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source,
-                Func<TSource, IEnumerable<TResult>> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-                foreach (TResult item in selector(element))
-                    yield return item;
-        }
-
-
-        public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source,
-                Func<TSource, int, IEnumerable<TResult>> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            int counter = 0;
-            foreach (TSource element in source)
-            {
-                foreach (TResult item in selector(element, counter))
-                    yield return item;
-                counter++;
-            }
-        }
-
-        #endregion
-
-        #region Single
-
-        public static TSource Single<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool otherElement = false;
-            TSource singleElement = default(TSource);
-            foreach (TSource element in source)
-            {
-                if (otherElement) throw new InvalidOperationException();
-                if (!otherElement) otherElement = true;
-                singleElement = element;
-            }
-
-            if (otherElement)
-                return singleElement;
-            else
-                throw new InvalidOperationException();
-        }
-
-
-        public static TSource Single<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            bool otherElement = false;
-            TSource singleElement = default(TSource);
-            foreach (TSource element in source)
-            {
-                if (predicate(element))
-                {
-                    if (otherElement) throw new InvalidOperationException();
-                    if (!otherElement) otherElement = true;
-                    singleElement = element;
-                }
-            }
-
-            if (otherElement)
-                return singleElement;
-            else
-                throw new InvalidOperationException();
-        }
-
-        #endregion
-
-        #region SingleOrDefault
-
-        public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            bool otherElement = false;
-            TSource singleElement = default(TSource);
-            foreach (TSource element in source)
-            {
-                if (otherElement) throw new InvalidOperationException();
-                if (!otherElement) otherElement = true;
-                singleElement = element;
-            }
-
-            return singleElement;
-        }
-
-
-        public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            bool otherElement = false;
-            TSource singleElement = default(TSource);
-            foreach (TSource element in source)
-            {
-                if (predicate(element))
-                {
-                    if (otherElement) throw new InvalidOperationException();
-                    if (!otherElement) otherElement = true;
-                    singleElement = element;
-                }
-            }
-
-            return singleElement;
-        }
-
-        #endregion
-
-        #region Skip
-        public static IEnumerable<TSource> Skip<TSource>(this IEnumerable<TSource> source, int count)
-        {
-            if (source == null)
-                throw new NotSupportedException();
-
-            int i = 0;
-            foreach (TSource e in source)
-            {
-                if (++i < count)
-                    continue;
-                yield return e;
-            }
-        }
-        #endregion
-
-        #region SkipWhile
-
-
-        public static IEnumerable<T> SkipWhile<T>(
-                IEnumerable<T> source,
-                Func<T, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            bool yield = false;
-
-            foreach (T element in source)
-            {
-                if (yield)
-                    yield return element;
-                else
-                    if (!predicate(element))
-                    {
-                        yield return element;
-                        yield = true;
-                    }
-            }
-        }
-
-
-        public static IEnumerable<TSource> SkipWhile<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, int, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            int counter = 0;
-            bool yield = false;
-
-            foreach (TSource element in source)
-            {
-                if (yield)
-                    yield return element;
-                else
-                    if (!predicate(element, counter))
-                    {
-                        yield return element;
-                        yield = true;
-                    }
-                counter++;
-            }
-        }
-
-        #endregion
-
-        #region Sum
-
-        public static int Sum(this IEnumerable<int> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException("source");
-
-            int sum = 0;
-            foreach (int element in source)
-                sum += element;
-
-            return sum;
-        }
-
-
-        public static int Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            int sum = 0;
-            foreach (TSource element in source)
-                sum += selector(element);
-
-            return sum;
-        }
-
-
-        public static int? Sum(this IEnumerable<int?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            int? sum = 0;
-            foreach (int? element in source)
-                if (element.HasValue)
-                    sum += element.Value;
-
-            return sum;
-        }
-
-
-        public static int? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            int? sum = 0;
-            foreach (TSource element in source)
-            {
-                int? item = selector(element);
-                if (item.HasValue)
-                    sum += item.Value;
-            }
-
-            return sum;
-        }
-
-
-        public static long Sum(this IEnumerable<long> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            long sum = 0;
-            foreach (long element in source)
-                sum += element;
-
-            return sum;
-        }
-
-
-        public static long Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            long sum = 0;
-            foreach (TSource element in source)
-                sum += selector(element);
-
-            return sum;
-        }
-
-
-        public static long? Sum(this IEnumerable<long?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            long? sum = 0;
-            foreach (long? element in source)
-                if (element.HasValue)
-                    sum += element.Value;
-
-            return sum;
-        }
-
-
-        public static long? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            long? sum = 0;
-            foreach (TSource element in source)
-            {
-                long? item = selector(element);
-                if (item.HasValue)
-                    sum += item.Value;
-            }
-
-            return sum;
-        }
-
-
-        public static double Sum(this IEnumerable<double> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            double sum = 0;
-            foreach (double element in source)
-                sum += element;
-
-            return sum;
-        }
-
-
-        public static double Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            double sum = 0;
-            foreach (TSource element in source)
-                sum += selector(element);
-
-            return sum;
-        }
-
-
-        public static double? Sum(this IEnumerable<double?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            double? sum = 0;
-            foreach (double? element in source)
-                if (element.HasValue)
-                    sum += element.Value;
-
-            return sum;
-        }
-
-
-        public static double? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            double? sum = 0;
-            foreach (TSource element in source)
-            {
-                double? item = selector(element);
-                if (item.HasValue)
-                    sum += item.Value;
-            }
-
-            return sum;
-        }
-
-
-        public static decimal Sum(this IEnumerable<decimal> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            decimal sum = 0;
-            foreach (decimal element in source)
-                sum += element;
-
-            return sum;
-        }
-
-
-        public static decimal Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            decimal sum = 0;
-            foreach (TSource element in source)
-                sum += selector(element);
-
-            return sum;
-        }
-
-
-        public static decimal? Sum(this IEnumerable<decimal?> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            decimal? sum = 0;
-            foreach (decimal? element in source)
-                if (element.HasValue)
-                    sum += element.Value;
-
-            return sum;
-        }
-
-
-        public static decimal? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
-        {
-            if (source == null || selector == null)
-                throw new ArgumentNullException();
-
-            decimal? sum = 0;
-            foreach (TSource element in source)
-            {
-                decimal? item = selector(element);
-                if (item.HasValue)
-                    sum += item.Value;
-            }
-
-            return sum;
-        }
-
-        #endregion
-
        #region Take
-
-        public static IEnumerable<T> Take<T>(this IEnumerable<T> source, int count)
-        {
-            if (source == null)
-                throw new ArgumentNullException();
-
-            if (count <= 0)
-                yield break;
-            else
-            {
-                int counter = 0;
-                foreach (T element in source)
-                {
-                    yield return element;
-                    counter++;
-                    if (counter == count)
-                        yield break;
-                }
-            }
-        }
-
-        #endregion
-
-        #region TakeWhile
-
-        public static IEnumerable<T> TakeWhile<T>(this IEnumerable<T> source, Func<T, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            foreach (T element in source)
-            {
-                if (predicate(element))
-                    yield return element;
-                else
-                    yield break;
-            }
-        }
-
-        public static IEnumerable<TSource> TakeWhile<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            int counter = 0;
-            foreach (TSource element in source)
-            {
-                if (predicate(element, counter))
-                    yield return element;
-                else
-                    yield break;
-                counter++;
-            }
-        }
-
-        #endregion
-
-        #region ThenBy
-
-        public static OrderedSequence<TSource> ThenBy<TSource, TKey>(this OrderedSequence<TSource> source, Func<TSource, TKey> keySelector)
-        {
-            return ThenBy<TSource, TKey>(source, keySelector, null);
-        }
-
-
-        public static OrderedSequence<TSource> ThenBy<TSource, TKey>(this OrderedSequence<TSource> source, 
-            Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null)
-                throw new ArgumentNullException();
-
-            return new InternalOrderedSequence<TSource, TKey>(
-                    source, keySelector, (comparer ?? Comparer<TKey>.Default), false, source);
-        }
-
-        #endregion
-
-        #region ThenByDescending
-
-        public static OrderedSequence<TSource> ThenByDescending<TSource, TKey>(this OrderedSequence<TSource> source,
-            Func<TSource, TKey> keySelector)
-        {
-            return ThenByDescending<TSource, TKey>(source, keySelector, null);
-        }
-
-
-        public static OrderedSequence<TSource> ThenByDescending<TSource, TKey>(this OrderedSequence<TSource> source,
-            Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null)
-                throw new ArgumentNullException();
-
-            return new InternalOrderedSequence<TSource, TKey>(
-                    source, keySelector, (comparer ?? Comparer<TKey>.Default), true, source);
-        }
-
-        #endregion
-
-        #region ToArray               
-        public static T[] ToArray<T> (this IEnumerable<T> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException ();
-                        
-            List<T> list = new List<T> (source);
-            return list.ToArray ();
-        }
-                
-        #endregion
-
-        #region ToDictionary
-        public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
-        {
-            return ToDictionary<TSource, TKey, TElement>(source, keySelector, elementSelector, null);
-        }
-
-
-        public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
-        {
-            if (source == null)
-                throw new ArgumentNullException("source");
-            if (keySelector == null)
-                throw new ArgumentNullException("keySelector");
-            if (elementSelector == null)
-                throw new ArgumentNullException("elementSelector");
-
-            Dictionary<TKey, TElement> dict = new Dictionary<TKey, TElement>(comparer);
-            foreach (TSource e in source)
-            {
-                dict.Add(keySelector(e), elementSelector(e));
-            }
-
-            return dict;
-        }
-        #endregion
-
-        #region ToList
-        public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException("source");
-
-            return new List<TSource>(source);
-        }
-        #endregion
-
-        #region ToLookup
-
-        public static Lookup<TKey, TSource> ToLookup<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
-        {
-            return ToLookup<TSource, TKey>(source, keySelector, null);
-        }
-
-
-        public static Lookup<TKey, TSource> ToLookup<TSource, TKey>(this IEnumerable<TSource> source, 
-            Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null)
-                throw new ArgumentNullException();
-
-            Dictionary<TKey, List<TSource>> dictionary = new Dictionary<TKey, List<TSource>>(comparer ?? EqualityComparer<TKey>.Default);
-            foreach (TSource element in source)
-            {
-                TKey key = keySelector(element);
-                if (key == null)
-                    throw new ArgumentNullException();
-                if (!dictionary.ContainsKey(key))
-                    dictionary.Add(key, new List<TSource>());
-                dictionary[key].Add(element);
-            }
-            return new Lookup<TKey, TSource>(dictionary);
-        }
-
-
-        public static Lookup<TKey, TElement> ToLookup<TSource, TKey, TElement>(this IEnumerable<TSource> source,
-            Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
-        {
-            return ToLookup<TSource, TKey, TElement>(source, keySelector, elementSelector, null);
-        }
-
-
-        public static Lookup<TKey, TElement> ToLookup<TSource, TKey, TElement>(this IEnumerable<TSource> source,
-            Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
-        {
-            if (source == null || keySelector == null || elementSelector == null)
-                throw new ArgumentNullException();
-
-            Dictionary<TKey, List<TElement>> dictionary = new Dictionary<TKey, List<TElement>>(comparer ?? EqualityComparer<TKey>.Default);
-            foreach (TSource element in source)
-            {
-                TKey key = keySelector(element);
-                if (key == null)
-                    throw new ArgumentNullException();
-                if (!dictionary.ContainsKey(key))
-                    dictionary.Add(key, new List<TElement>());
-                dictionary[key].Add(elementSelector(element));
-            }
-            return new Lookup<TKey, TElement>(dictionary);
-        }
-
-        #endregion
-
-        #region ToSequence
-
-        public static IEnumerable<T> ToSequence<T>(this IEnumerable<T> source)
-        {
-            return (IEnumerable<T>)source;
-        }
-
-        #endregion
-
-        #region Union
-
-
-        public static IEnumerable<T> Union<T>(this IEnumerable<T> first, IEnumerable<T> second)
-        {
-            if (first == null || second == null)
-                throw new ArgumentNullException();
-
-            List<T> items = new List<T>();
-            foreach (T element in first)
-            {
-                if (IndexOf(items, element) == -1)
-                {
-                    items.Add(element);
-                    yield return element;
-                }
-            }
-            foreach (T element in second)
-            {
-                if (IndexOf(items, element) == -1)
-                {
-                    items.Add(element);
-                    yield return element;
-                }
-            }
-        }
-
-        #endregion
-
-        #region Where
-
-        public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            foreach (TSource element in source)
-                if (predicate(element))
-                    yield return element;
-        }
-
-
-        public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source,
-                Func<TSource, int, bool> predicate)
-        {
-            if (source == null || predicate == null)
-                throw new ArgumentNullException();
-
-            int counter = 0;
-            foreach (TSource element in source)
-            {
-                if (predicate(element, counter))
-                    yield return element;
-                counter++;
-            }
-        }
-
-        #endregion
-
-        // These methods are not included in the
-        // .NET Standard Query Operators Specification,
-        // but they provide additional useful commands
-
-        #region Compare
-
-        private static bool Equals<T>(T first, T second)
-        {
-            // Mostly, values in Enumerable<T> 
-            // sequences need to be compared using
-            // Equals and GetHashCode
-
-            if (first == null || second == null)
-                return (first == null && second == null);
-            else
-                return ((first.Equals(second) ||
-                         first.GetHashCode() == second.GetHashCode()));
-        }
-
-        #endregion
-
-        #region IndexOf
-
-        public static int IndexOf<T>(this IEnumerable<T> source, T item, IEqualityComparer<T> comparer)
-        {
-            if (comparer == null)
-                comparer = EqualityComparer<T>.Default;
-
-            int counter = 0;
-            foreach (T element in source)
-            {
-                if (comparer.Equals(element, item))
-                    return counter;
-                counter++;
-            }
-            // The item was not found
-            return -1;
-        }
-
-        public static int IndexOf<T>(this IEnumerable<T> source, T item)
-        {
-            return IndexOf<T>(source, item, null);
-        }
-        #endregion
-
-        #region ToReadOnlyCollection
-        internal static ReadOnlyCollection<TSource> ToReadOnlyCollection<TSource>(IEnumerable<TSource> source)
-        {
-            if (source == null)
-                return new ReadOnlyCollection<TSource>(new List<TSource>());
-
-            if (typeof(ReadOnlyCollection<TSource>).IsInstanceOfType(source))
-                return source as ReadOnlyCollection<TSource>;
-
-            return new ReadOnlyCollection<TSource>(ToArray<TSource>(source));
-        }
-        #endregion
-    }
+//
+// Enumerable.cs
+//
+// Authors:
+//  Marek Safar (marek.safar@gmail.com)
+//  Antonello Provenzano  <antonello@deveel.com>
+//  Alejandro Serrano "Serras" (trupill@yahoo.es)
+//  Jb Evain (jbevain@novell.com)
+//
+// Copyright (C) 2007 Novell, Inc (http://www.novell.com)
+//
+// Permission is hereby granted, free of charge, to any person obtaining
+// a copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//
+
+// precious: http://www.hookedonlinq.com
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+
+namespace System.Linq
+{
+	public static class Enumerable
+	{
+		enum Fallback {
+			Default,
+			Throw
+		}
+
+		class PredicateOf<T> {
+			public static readonly Func<T, bool> Always = (t) => true;
+		}
+
+		class Function<T> {
+			public static readonly Func<T, T> Identity = (t) => t;
+		}
+
+		#region Aggregate
+
+		public static TSource Aggregate<TSource> (this IEnumerable<TSource> source, Func<TSource, TSource, TSource> func)
+		{
+			Check.SourceAndFunc (source, func);
+
+			// custom foreach so that we can efficiently throw an exception
+			// if zero elements and treat the first element differently
+			using (var enumerator = source.GetEnumerator ()) {
+				if (!enumerator.MoveNext ())
+					throw new InvalidOperationException ("No elements in source list");
+
+				TSource folded = enumerator.Current;
+				while (enumerator.MoveNext ())
+					folded = func (folded, enumerator.Current);
+				return folded;
+			}
+		}
+
+		public static TAccumulate Aggregate<TSource, TAccumulate> (this IEnumerable<TSource> source,
+			TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func)
+		{
+			Check.SourceAndFunc (source, func);
+
+			TAccumulate folded = seed;
+			foreach (TSource element in source)
+				folded = func (folded, element);
+
+			return folded;
+		}
+
+		public static TResult Aggregate<TSource, TAccumulate, TResult> (this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector)
+		{
+			Check.SourceAndFunc (source, func);
+			if (resultSelector == null)
+				throw new ArgumentNullException ("resultSelector");
+
+			var result = seed;
+			foreach (var e in source)
+				result = func (result, e);
+
+			return resultSelector (result);
+		}
+
+		#endregion
+
+		#region All
+
+		public static bool All<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			foreach (var element in source)
+				if (!predicate (element))
+					return false;
+
+			return true;
+		}
+
+		#endregion
+
+		#region Any
+
+		public static bool Any<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			var collection = source as ICollection<TSource>;
+			if (collection != null)
+				return collection.Count > 0;
+
+			using (var enumerator = source.GetEnumerator ())
+				return enumerator.MoveNext ();
+		}
+
+		public static bool Any<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			foreach (TSource element in source)
+				if (predicate (element))
+					return true;
+
+			return false;
+		}
+
+		#endregion
+
+		#region AsEnumerable
+
+		public static IEnumerable<TSource> AsEnumerable<TSource> (this IEnumerable<TSource> source)
+		{
+			return source;
+		}
+
+		#endregion
+
+		#region Average
+
+		public static double Average (this IEnumerable<int> source)
+		{
+			return Average<int, long, double> (source, (a, b) => a + b, (a, b) => (double) a / (double) b);
+		}
+
+		public static double Average (this IEnumerable<long> source)
+		{
+			return Average<long, long, double> (source, (a, b) => a + b, (a, b) => (double) a / (double) b);
+		}
+
+		public static double Average (this IEnumerable<double> source)
+		{
+			return Average<double, double, double> (source, (a, b) => a + b, (a, b) => a / b);
+		}
+
+		public static float Average (this IEnumerable<float> source)
+		{
+			return Average<float, double, float> (source, (a, b) => a + b, (a, b) => (float) a / (float) b);
+		}
+
+		public static decimal Average (this IEnumerable<decimal> source)
+		{
+			return Average<decimal, decimal, decimal> (source, (a, b) => a + b, (a, b) => a / b);
+		}
+
+		static TResult Average<TElement, TAggregate, TResult> (this IEnumerable<TElement> source,
+			Func<TAggregate, TElement, TAggregate> func, Func<TAggregate, long, TResult> result)
+			where TElement : struct
+			where TAggregate : struct
+			where TResult : struct
+		{
+			Check.Source (source);
+
+			var total = default (TAggregate);
+			long counter = 0;
+			foreach (var element in source) {
+				total = func (total, element);
+				++counter;
+			}
+
+			if (counter == 0)
+				throw new InvalidOperationException ();
+
+			return result (total, counter);
+		}
+
+		static TResult? AverageNullable<TElement, TAggregate, TResult> (this IEnumerable<TElement?> source,
+			Func<TAggregate, TElement, TAggregate> func, Func<TAggregate, long, TResult> result)
+			where TElement : struct
+			where TAggregate : struct
+			where TResult : struct
+		{
+			Check.Source (source);
+
+			var total = default (TAggregate);
+			long counter = 0;
+			foreach (var element in source) {
+				if (!element.HasValue)
+					continue;
+
+				total = func (total, element.Value);
+				counter++;
+			}
+
+			if (counter == 0)
+				return null;
+
+			return new TResult? (result (total, counter));
+		}
+
+		public static double? Average (this IEnumerable<int?> source)
+		{
+			Check.Source (source);
+
+			return source.AverageNullable<int, long, double> ((a, b) => a + b, (a, b) => (double) a / (double) b);
+		}
+
+		public static double? Average (this IEnumerable<long?> source)
+		{
+			Check.Source (source);
+
+			return source.AverageNullable<long, long, double> ((a, b) => a + b, (a, b) => (double) a / b);
+		}
+
+		public static double? Average (this IEnumerable<double?> source)
+		{
+			Check.Source (source);
+
+			return source.AverageNullable<double, double, double> ((a, b) => a + b, (a, b) => a / b);
+		}
+
+		public static decimal? Average (this IEnumerable<decimal?> source)
+		{
+			Check.Source (source);
+
+			return source.AverageNullable<decimal, decimal, decimal> ((a, b) => a + b, (a, b) => a / b);
+		}
+
+		public static float? Average (this IEnumerable<float?> source)
+		{
+			Check.Source (source);
+
+			return source.AverageNullable<float, double, float> ((a, b) => a + b, (a, b) => (float) a / (float) b);
+		}
+
+		public static double Average<TSource> (this IEnumerable<TSource> source, Func<TSource, int> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).Average<int, long, double> ((a, b) => a + b, (a, b) => (double) a / (double) b);
+		}
+
+		public static double? Average<TSource> (this IEnumerable<TSource> source, Func<TSource, int?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).AverageNullable<int, long, double> ((a, b) => a + b, (a, b) => (double) a / (double) b);
+		}
+
+		public static double Average<TSource> (this IEnumerable<TSource> source, Func<TSource, long> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).Average<long, long, double> ((a, b) => a + b, (a, b) => (double) a / (double) b);
+		}
+
+		public static double? Average<TSource> (this IEnumerable<TSource> source, Func<TSource, long?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).AverageNullable<long, long, double> ((a, b) => a + b, (a, b) => (double) a / (double) b);
+		}
+
+		public static double Average<TSource> (this IEnumerable<TSource> source, Func<TSource, double> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).Average<double, double, double> ((a, b) => a + b, (a, b) => a / b);
+		}
+
+		public static double? Average<TSource> (this IEnumerable<TSource> source, Func<TSource, double?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).AverageNullable<double, double, double> ((a, b) => a + b, (a, b) => a / b);
+		}
+
+		public static float Average<TSource> (this IEnumerable<TSource> source, Func<TSource, float> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).Average<float, double, float> ((a, b) => a + b, (a, b) => (float) a / (float) b);
+		}
+
+		public static float? Average<TSource> (this IEnumerable<TSource> source, Func<TSource, float?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).AverageNullable<float, double, float> ((a, b) => a + b, (a, b) => (float) a / (float) b);
+		}
+
+		public static decimal Average<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).Average<decimal, decimal, decimal> ((a, b) => a + b, (a, b) => a / b);
+		}
+
+		public static decimal? Average<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.Select (selector).AverageNullable<decimal, decimal, decimal> ((a, b) => a + b, (a, b) => a / b);
+		}
+
+		#endregion
+
+		#region Cast
+
+		public static IEnumerable<TResult> Cast<TResult> (this IEnumerable source)
+		{
+			Check.Source (source);
+
+			return CreateCastIterator<TResult> (source);
+		}
+
+		static IEnumerable<TResult> CreateCastIterator<TResult> (IEnumerable source)
+		{
+			foreach (TResult element in source)
+				yield return element;
+		}
+
+		#endregion
+
+		#region Concat
+
+		public static IEnumerable<TSource> Concat<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second)
+		{
+			Check.FirstAndSecond (first, second);
+
+			return CreateConcatIterator (first, second);
+		}
+
+		static IEnumerable<TSource> CreateConcatIterator<TSource> (IEnumerable<TSource> first, IEnumerable<TSource> second)
+		{
+			foreach (TSource element in first)
+				yield return element;
+			foreach (TSource element in second)
+				yield return element;
+		}
+
+		#endregion
+
+		#region Contains
+
+		public static bool Contains<TSource> (this IEnumerable<TSource> source, TSource value)
+		{
+			var collection = source as ICollection<TSource>;
+			if (collection != null)
+				return collection.Contains (value);
+
+			return Contains<TSource> (source, value, null);
+		}
+
+		public static bool Contains<TSource> (this IEnumerable<TSource> source, TSource value, IEqualityComparer<TSource> comparer)
+		{
+			Check.Source (source);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TSource>.Default;
+
+			foreach (var element in source)
+				if (comparer.Equals (element, value))
+					return true;
+
+			return false;
+		}
+		#endregion
+
+		#region Count
+
+		public static int Count<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			var collection = source as ICollection<TSource>;
+			if (collection != null)
+				return collection.Count;
+
+			int counter = 0;
+			using (var enumerator = source.GetEnumerator ())
+				while (enumerator.MoveNext ())
+					counter++;
+
+			return counter;
+		}
+
+		public static int Count<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			int counter = 0;
+			foreach (var element in source)
+				if (selector (element))
+					counter++;
+
+			return counter;
+		}
+
+		#endregion
+
+		#region DefaultIfEmpty
+
+		public static IEnumerable<TSource> DefaultIfEmpty<TSource> (this IEnumerable<TSource> source)
+		{
+			return DefaultIfEmpty (source, default (TSource));
+		}
+
+		public static IEnumerable<TSource> DefaultIfEmpty<TSource> (this IEnumerable<TSource> source, TSource defaultValue)
+		{
+			Check.Source (source);
+
+			return CreateDefaultIfEmptyIterator (source, defaultValue);
+		}
+
+		static IEnumerable<TSource> CreateDefaultIfEmptyIterator<TSource> (IEnumerable<TSource> source, TSource defaultValue)
+		{
+			bool empty = true;
+			foreach (TSource item in source) {
+				empty = false;
+				yield return item;
+			}
+
+			if (empty)
+				yield return defaultValue;
+		}
+
+		#endregion
+
+		#region Distinct
+
+		public static IEnumerable<TSource> Distinct<TSource> (this IEnumerable<TSource> source)
+		{
+			return Distinct<TSource> (source, null);
+		}
+
+		public static IEnumerable<TSource> Distinct<TSource> (this IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
+		{
+			Check.Source (source);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TSource>.Default;
+
+			return CreateDistinctIterator (source, comparer);
+		}
+
+		static IEnumerable<TSource> CreateDistinctIterator<TSource> (IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
+		{
+			var items = new HashSet<TSource> (comparer);
+			foreach (var element in source) {
+				if (! items.Contains (element)) {
+					items.Add (element);
+					yield return element;
+				}
+			}
+		}
+
+		#endregion
+
+		#region ElementAt
+
+		static TSource ElementAt<TSource> (this IEnumerable<TSource> source, int index, Fallback fallback)
+		{
+			long counter = 0L;
+
+			foreach (var element in source) {
+				if (index == counter++)
+					return element;
+			}
+
+			if (fallback == Fallback.Throw)
+				throw new ArgumentOutOfRangeException ();
+
+			return default (TSource);
+		}
+
+		public static TSource ElementAt<TSource> (this IEnumerable<TSource> source, int index)
+		{
+			Check.Source (source);
+
+			if (index < 0)
+				throw new ArgumentOutOfRangeException ();
+
+			var list = source as IList<TSource>;
+			if (list != null)
+				return list [index];
+
+			return source.ElementAt (index, Fallback.Throw);
+		}
+
+		#endregion
+
+		#region ElementAtOrDefault
+
+		public static TSource ElementAtOrDefault<TSource> (this IEnumerable<TSource> source, int index)
+		{
+			Check.Source (source);
+
+			if (index < 0)
+				return default (TSource);
+
+			var list = source as IList<TSource>;
+			if (list != null)
+				return index < list.Count ? list [index] : default (TSource);
+
+			return source.ElementAt (index, Fallback.Default);
+		}
+
+		#endregion
+
+		#region Empty
+
+		public static IEnumerable<TResult> Empty<TResult> ()
+		{
+			return new TResult [0];
+		}
+
+		#endregion
+
+		#region Except
+
+		public static IEnumerable<TSource> Except<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second)
+		{
+			return Except (first, second, null);
+		}
+
+		public static IEnumerable<TSource> Except<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+		{
+			Check.FirstAndSecond (first, second);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TSource>.Default;
+
+			return CreateExceptIterator (first, second, comparer);
+		}
+
+		static IEnumerable<TSource> CreateExceptIterator<TSource> (IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+		{
+			var items = new HashSet<TSource> (second, comparer);
+			foreach (var element in first) {
+				if (!items.Contains (element, comparer))
+					yield return element;
+			}
+		}
+
+		#endregion
+
+		#region First
+
+		static TSource First<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate, Fallback fallback)
+		{
+			foreach (var element in source)
+				if (predicate (element))
+					return element;
+
+			if (fallback == Fallback.Throw)
+				throw new InvalidOperationException ();
+
+			return default (TSource);
+		}
+
+		public static TSource First<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			var list = source as IList<TSource>;
+			if (list != null) {
+				if (list.Count != 0)
+					return list [0];
+
+				throw new InvalidOperationException ();
+			} else {
+				using (var enumerator = source.GetEnumerator ()) {
+					if (enumerator.MoveNext ())
+						return enumerator.Current;
+				}
+			}
+
+			throw new InvalidOperationException ();
+		}
+
+		public static TSource First<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return source.First (predicate, Fallback.Throw);
+		}
+
+		#endregion
+
+		#region FirstOrDefault
+
+		public static TSource FirstOrDefault<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			return source.First (PredicateOf<TSource>.Always, Fallback.Default);
+		}
+
+		public static TSource FirstOrDefault<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return source.First (predicate, Fallback.Default);
+		}
+
+		#endregion
+
+		#region GroupBy
+
+		private static List<T> ContainsGroup<K, T> (
+				Dictionary<K, List<T>> items, K key, IEqualityComparer<K> comparer)
+		{
+			IEqualityComparer<K> comparerInUse = (comparer ?? EqualityComparer<K>.Default);
+			foreach (KeyValuePair<K, List<T>> value in items) {
+				if (comparerInUse.Equals (value.Key, key))
+					return value.Value;
+			}
+			return null;
+		}
+
+		public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector)
+		{
+			return GroupBy<TSource, TKey> (source, keySelector, null);
+		}
+
+		public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
+		{
+			Check.SourceAndKeySelector (source, keySelector);
+
+			return CreateGroupByIterator (source, keySelector, comparer);
+		}
+
+		static IEnumerable<IGrouping<TKey, TSource>> CreateGroupByIterator<TSource, TKey> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
+		{
+			Dictionary<TKey, List<TSource>> groups = new Dictionary<TKey, List<TSource>> ();
+			List<TSource> nullList = new List<TSource> ();
+			int counter = 0;
+			int nullCounter = -1;
+
+			foreach (TSource element in source) {
+				TKey key = keySelector (element);
+				if (key == null) {
+					nullList.Add (element);
+					if (nullCounter == -1) {
+						nullCounter = counter;
+						counter++;
+					}
+				} else {
+					List<TSource> group = ContainsGroup<TKey, TSource> (groups, key, comparer);
+					if (group == null) {
+						group = new List<TSource> ();
+						groups.Add (key, group);
+						counter++;
+					}
+					group.Add (element);
+				}
+			}
+
+			counter = 0;
+			foreach (KeyValuePair<TKey, List<TSource>> group in groups) {
+				if (counter == nullCounter) {
+					Grouping<TKey, TSource> nullGroup = new Grouping<TKey, TSource> (default (TKey), nullList);
+					yield return nullGroup;
+					counter++;
+				}
+				Grouping<TKey, TSource> grouping = new Grouping<TKey, TSource> (group.Key, group.Value);
+				yield return grouping;
+				counter++;
+			}
+		}
+
+		public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
+		{
+			return GroupBy<TSource, TKey, TElement> (source, keySelector, elementSelector, null);
+		}
+
+		public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
+		{
+			Check.SourceAndKeyElementSelectors (source, keySelector, elementSelector);
+
+			Dictionary<TKey, List<TElement>> groups = new Dictionary<TKey, List<TElement>> ();
+			List<TElement> nullList = new List<TElement> ();
+			int counter = 0;
+			int nullCounter = -1;
+
+			foreach (TSource item in source) {
+				TKey key = keySelector (item);
+				TElement element = elementSelector (item);
+				if (key == null) {
+					nullList.Add (element);
+					if (nullCounter == -1) {
+						nullCounter = counter;
+						counter++;
+					}
+				} else {
+					List<TElement> group = ContainsGroup<TKey, TElement> (groups, key, comparer);
+					if (group == null) {
+						group = new List<TElement> ();
+						groups.Add (key, group);
+						counter++;
+					}
+					group.Add (element);
+				}
+			}
+
+			counter = 0;
+			foreach (KeyValuePair<TKey, List<TElement>> group in groups) {
+				if (counter == nullCounter) {
+					Grouping<TKey, TElement> nullGroup = new Grouping<TKey, TElement> (default (TKey), nullList);
+					yield return nullGroup;
+					counter++;
+				}
+				Grouping<TKey, TElement> grouping = new Grouping<TKey, TElement> (group.Key, group.Value);
+				yield return grouping;
+				counter++;
+			}
+		}
+
+		public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector,
+			Func<TKey, IEnumerable<TElement>, TResult> resultSelector)
+		{
+			return GroupBy (source, keySelector, elementSelector, resultSelector, null);
+		}
+
+		public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector,
+			Func<TKey, IEnumerable<TElement>, TResult> resultSelector,
+			IEqualityComparer<TKey> comparer)
+		{
+			IEnumerable<IGrouping<TKey, TElement>> groups = GroupBy<TSource, TKey, TElement> (
+				source, keySelector, elementSelector, comparer);
+
+			foreach (IGrouping<TKey, TElement> group in groups)
+				yield return resultSelector (group.Key, group);
+		}
+
+		public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector,
+			Func<TKey, IEnumerable<TSource>, TResult> resultSelector)
+		{
+			return GroupBy (source, keySelector, resultSelector, null);
+		}
+
+		public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector,
+			Func<TKey, IEnumerable<TSource>, TResult> resultSelector,
+			IEqualityComparer<TKey> comparer)
+		{
+			IEnumerable<IGrouping<TKey,TSource>> groups = GroupBy<TSource, TKey> (source, keySelector, comparer);
+
+			foreach (IGrouping<TKey, TSource> group in groups)
+				yield return resultSelector (group.Key, group);
+		}
+
+		#endregion
+
+		# region GroupJoin
+
+		public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult> (this IEnumerable<TOuter> outer,
+			IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
+			Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector)
+		{
+			return GroupJoin (outer, inner, outerKeySelector, innerKeySelector, resultSelector, null);
+		}
+
+		public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult> (this IEnumerable<TOuter> outer,
+			IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
+			Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector,
+			IEqualityComparer<TKey> comparer)
+		{
+			Check.JoinSelectors (outer, inner, outerKeySelector, innerKeySelector, resultSelector);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TKey>.Default;
+
+			return CreateGroupJoinIterator (outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer);
+		}
+
+		static IEnumerable<TResult> CreateGroupJoinIterator<TOuter, TInner, TKey, TResult> (this IEnumerable<TOuter> outer,
+			IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
+			Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector,
+			IEqualityComparer<TKey> comparer)
+		{
+			ILookup<TKey, TInner> innerKeys = ToLookup<TInner, TKey> (inner, innerKeySelector, comparer);
+			/*Dictionary<K, List<U>> innerKeys = new Dictionary<K, List<U>> ();
+			foreach (U element in inner)
+			{
+					K innerKey = innerKeySelector (element);
+					if (!innerKeys.ContainsKey (innerKey))
+							innerKeys.Add (innerKey, new List<U> ());
+					innerKeys[innerKey].Add (element);
+			}*/
+
+			foreach (TOuter element in outer) {
+				TKey outerKey = outerKeySelector (element);
+				if (innerKeys.Contains (outerKey))
+					yield return resultSelector (element, innerKeys [outerKey]);
+				else
+					yield return resultSelector (element, Empty<TInner> ());
+			}
+		}
+
+		#endregion
+
+		#region Intersect
+
+		public static IEnumerable<TSource> Intersect<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second)
+		{
+			return Intersect (first, second, null);
+		}
+
+		public static IEnumerable<TSource> Intersect<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+		{
+			Check.FirstAndSecond (first, second);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TSource>.Default;
+
+			return CreateIntersectIterator (first, second, comparer);
+		}
+
+		static IEnumerable<TSource> CreateIntersectIterator<TSource> (IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+		{
+			var items = new HashSet<TSource> (second, comparer);
+			foreach (TSource element in first) {
+				if (items.Remove (element))
+					yield return element;
+			}
+		}
+
+		#endregion
+
+		# region Join
+
+		public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult> (this IEnumerable<TOuter> outer,
+			IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
+			Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector, IEqualityComparer<TKey> comparer)
+		{
+			Check.JoinSelectors (outer, inner, outerKeySelector, innerKeySelector, resultSelector);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TKey>.Default;
+
+			return CreateJoinIterator (outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer);
+		}
+
+		static IEnumerable<TResult> CreateJoinIterator<TOuter, TInner, TKey, TResult> (this IEnumerable<TOuter> outer,
+			IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
+			Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector, IEqualityComparer<TKey> comparer)
+		{
+			ILookup<TKey, TInner> innerKeys = ToLookup<TInner, TKey> (inner, innerKeySelector, comparer);
+			/*Dictionary<K, List<U>> innerKeys = new Dictionary<K, List<U>> ();
+			foreach (U element in inner)
+			{
+					K innerKey = innerKeySelector (element);
+					if (!innerKeys.ContainsKey (innerKey))
+							innerKeys.Add (innerKey, new List<U> ());
+					innerKeys[innerKey].Add (element);
+			}*/
+
+			foreach (TOuter element in outer) {
+				TKey outerKey = outerKeySelector (element);
+				if (innerKeys.Contains (outerKey)) {
+					foreach (TInner innerElement in innerKeys [outerKey])
+						yield return resultSelector (element, innerElement);
+				}
+			}
+		}
+
+		public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult> (this IEnumerable<TOuter> outer,
+			IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector,
+			Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector)
+		{
+			return outer.Join (inner, outerKeySelector, innerKeySelector, resultSelector, null);
+		}
+
+		#endregion
+
+		#region Last
+
+		static TSource Last<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate, Fallback fallback)
+		{
+			var empty = true;
+			var item = default (TSource);
+
+			foreach (var element in source) {
+				if (!predicate (element))
+					continue;
+
+				item = element;
+				empty = false;
+			}
+
+			if (!empty)
+				return item;
+
+			if (fallback == Fallback.Throw)
+				throw new InvalidOperationException ();
+
+			return item;
+		}
+
+		public static TSource Last<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			var collection = source as ICollection<TSource>;
+			if (collection != null && collection.Count == 0)
+				throw new InvalidOperationException ();
+
+			var list = source as IList<TSource>;
+			if (list != null)
+				return list [list.Count - 1];
+
+			return source.Last (PredicateOf<TSource>.Always, Fallback.Throw);
+		}
+
+		public static TSource Last<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return source.Last (predicate, Fallback.Throw);
+		}
+
+		#endregion
+
+		#region LastOrDefault
+
+		public static TSource LastOrDefault<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			var list = source as IList<TSource>;
+			if (list != null)
+				return list.Count > 0 ? list [list.Count - 1] : default (TSource);
+
+			return source.Last (PredicateOf<TSource>.Always, Fallback.Default);
+		}
+
+		public static TSource LastOrDefault<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return source.Last (predicate, Fallback.Default);
+		}
+
+		#endregion
+
+		#region LongCount
+
+		public static long LongCount<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+#if !NET_2_1
+			var array = source as TSource [];
+			if (array != null)
+				return array.LongLength;
+#endif
+
+			long counter = 0;
+			using (var enumerator = source.GetEnumerator ())
+				while (enumerator.MoveNext ())
+					counter++;
+
+			return counter;
+		}
+
+		public static long LongCount<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			long counter = 0;
+			foreach (TSource element in source)
+				if (selector (element))
+					counter++;
+
+			return counter;
+		}
+
+		#endregion
+
+		#region Max
+
+		public static int Max (this IEnumerable<int> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, int.MinValue, (a, b) => Math.Max (a, b));
+		}
+
+		public static long Max (this IEnumerable<long> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, long.MinValue, (a, b) => Math.Max (a, b));
+		}
+
+		public static double Max (this IEnumerable<double> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, double.MinValue, (a, b) => Math.Max (a, b));
+		}
+
+		public static float Max (this IEnumerable<float> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, float.MinValue, (a, b) => Math.Max (a, b));
+		}
+
+		public static decimal Max (this IEnumerable<decimal> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, decimal.MinValue, (a, b) => Math.Max (a, b));
+		}
+
+		public static int? Max (this IEnumerable<int?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Max (a, b));
+		}
+
+		public static long? Max (this IEnumerable<long?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Max (a, b));
+		}
+
+		public static double? Max (this IEnumerable<double?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Max (a, b));
+		}
+
+		public static float? Max (this IEnumerable<float?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Max (a, b));
+		}
+
+		public static decimal? Max (this IEnumerable<decimal?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Max (a, b));
+		}
+
+		static T? IterateNullable<T> (IEnumerable<T?> source, Func<T, T, T> selector) where T : struct
+		{
+			bool empty = true;
+			T? value = null;
+			foreach (var element in source) {
+				if (!element.HasValue)
+					continue;
+
+				if (!value.HasValue)
+					value = element.Value;
+				else
+					value = selector (element.Value, value.Value);
+
+				empty = false;
+			}
+
+			if (empty)
+				return null;
+
+			return value;
+		}
+
+		static TRet? IterateNullable<TSource, TRet> (
+			IEnumerable<TSource> source,
+			Func<TSource, TRet?> source_selector,
+			Func<TRet?, TRet?, bool> selector) where TRet : struct
+		{
+			bool empty = true;
+			TRet? value = null;
+			foreach (var element in source) {
+				TRet? item = source_selector (element);
+
+				if (!value.HasValue)
+					value = item;
+				else if (selector (item, value))
+					value = item;
+
+				empty = false;
+			}
+
+			if (empty)
+				return null;
+
+			return value;
+		}
+
+		public static TSource Max<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			bool notAssigned = true;
+			TSource maximum = default (TSource);
+			int counter = 0;
+			foreach (TSource element in source) {
+				if (notAssigned) {
+					maximum = element;
+					notAssigned = false;
+				} else {
+					int comparison;
+					if (element is IComparable<TSource>)
+						comparison = ((IComparable<TSource>) element).CompareTo (maximum);
+					else if (element is System.IComparable)
+						comparison = ((System.IComparable) element).CompareTo (maximum);
+					else
+						throw new ArgumentNullException ();
+
+					if (comparison > 0)
+						maximum = element;
+				}
+				counter++;
+			}
+
+			if (counter == 0)
+				throw new InvalidOperationException ();
+			else
+				return maximum;
+		}
+
+		public static int Max<TSource> (this IEnumerable<TSource> source, Func<TSource, int> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, int.MinValue, (a, b) => Math.Max (selector (a), b));
+		}
+
+		public static long Max<TSource> (this IEnumerable<TSource> source, Func<TSource, long> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, long.MinValue, (a, b) => Math.Max (selector (a), b));
+		}
+
+		public static double Max<TSource> (this IEnumerable<TSource> source, Func<TSource, double> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, double.MinValue, (a, b) => Math.Max (selector (a), b));
+		}
+
+		public static float Max<TSource> (this IEnumerable<TSource> source, Func<TSource, float> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, float.MinValue, (a, b) => Math.Max (selector (a), b));
+		}
+
+		public static decimal Max<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, decimal.MinValue, (a, b) => Math.Max (selector (a), b));
+		}
+
+		static U Iterate<T, U> (IEnumerable<T> source, U initValue, Func<T, U, U> selector)
+		{
+			bool empty = true;
+			foreach (var element in source) {
+				initValue = selector (element, initValue);
+				empty = false;
+			}
+
+			if (empty)
+				throw new InvalidOperationException ();
+
+			return initValue;
+		}
+
+		public static int? Max<TSource> (this IEnumerable<TSource> source, Func<TSource, int?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a > b);
+		}
+
+		public static long? Max<TSource> (this IEnumerable<TSource> source, Func<TSource, long?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a > b);
+		}
+
+		public static double? Max<TSource> (this IEnumerable<TSource> source, Func<TSource, double?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a > b);
+		}
+
+		public static float? Max<TSource> (this IEnumerable<TSource> source, Func<TSource, float?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a > b);
+		}
+
+		public static decimal? Max<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a > b);
+		}
+
+		public static TResult Max<TSource, TResult> (this IEnumerable<TSource> source, Func<TSource, TResult> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			bool notAssigned = true;
+			TResult maximum = default (TResult);
+			int counter = 0;
+			foreach (TSource item in source) {
+				TResult element = selector (item);
+				if (notAssigned) {
+					maximum = element;
+					notAssigned = false;
+				} else {
+					int comparison;
+					if (element is IComparable<TResult>)
+						comparison = ((IComparable<TResult>) element).CompareTo (maximum);
+					else if (element is System.IComparable)
+						comparison = ((System.IComparable) element).CompareTo (maximum);
+					else
+						throw new ArgumentNullException ();
+
+					if (comparison > 0)
+						maximum = element;
+				}
+				counter++;
+			}
+
+			if (counter == 0)
+				throw new InvalidOperationException ();
+			else
+				return maximum;
+		}
+
+		#endregion
+
+		#region Min
+
+		public static int Min (this IEnumerable<int> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, int.MaxValue, (a, b) => Math.Min (a, b));
+		}
+
+		public static long Min (this IEnumerable<long> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, long.MaxValue, (a, b) => Math.Min (a, b));
+		}
+
+		public static double Min (this IEnumerable<double> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, double.MaxValue, (a, b) => Math.Min (a, b));
+		}
+
+		public static float Min (this IEnumerable<float> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, float.MaxValue, (a, b) => Math.Min (a, b));
+		}
+
+		public static decimal Min (this IEnumerable<decimal> source)
+		{
+			Check.Source (source);
+
+			return Iterate (source, decimal.MaxValue, (a, b) => Math.Min (a, b));
+		}
+
+		public static int? Min (this IEnumerable<int?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Min (a, b));
+		}
+
+		public static long? Min (this IEnumerable<long?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Min (a, b));
+		}
+
+		public static double? Min (this IEnumerable<double?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Min (a, b));
+		}
+
+		public static float? Min (this IEnumerable<float?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Min (a, b));
+		}
+
+		public static decimal? Min (this IEnumerable<decimal?> source)
+		{
+			Check.Source (source);
+
+			return IterateNullable (source, (a, b) => Math.Min (a, b));
+		}
+
+		public static TSource Min<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			bool notAssigned = true;
+			TSource minimum = default (TSource);
+			int counter = 0;
+			foreach (TSource element in source) {
+				if (notAssigned) {
+					minimum = element;
+					notAssigned = false;
+				} else {
+					int comparison;
+					if (element is IComparable<TSource>)
+						comparison = ((IComparable<TSource>) element).CompareTo (minimum);
+					else if (element is System.IComparable)
+						comparison = ((System.IComparable) element).CompareTo (minimum);
+					else
+						throw new ArgumentNullException ();
+
+					if (comparison < 0)
+						minimum = element;
+				}
+				counter++;
+			}
+
+			if (counter == 0)
+				throw new InvalidOperationException ();
+			else
+				return minimum;
+		}
+
+		public static int Min<TSource> (this IEnumerable<TSource> source, Func<TSource, int> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, int.MaxValue, (a, b) => Math.Min (selector (a), b));
+		}
+
+		public static long Min<TSource> (this IEnumerable<TSource> source, Func<TSource, long> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, long.MaxValue, (a, b) => Math.Min (selector (a), b));
+		}
+
+		public static double Min<TSource> (this IEnumerable<TSource> source, Func<TSource, double> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, double.MaxValue, (a, b) => Math.Min (selector (a), b));
+		}
+
+		public static float Min<TSource> (this IEnumerable<TSource> source, Func<TSource, float> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, float.MaxValue, (a, b) => Math.Min (selector (a), b));
+		}
+
+		public static decimal Min<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Iterate (source, decimal.MaxValue, (a, b) => Math.Min (selector (a), b));
+		}
+
+		public static int? Min<TSource> (this IEnumerable<TSource> source, Func<TSource, int?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a < b);
+		}
+
+		public static long? Min<TSource> (this IEnumerable<TSource> source, Func<TSource, long?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a < b);
+		}
+
+		public static float? Min<TSource> (this IEnumerable<TSource> source, Func<TSource, float?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a < b);
+		}
+
+		public static double? Min<TSource> (this IEnumerable<TSource> source, Func<TSource, double?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a < b);
+		}
+
+		public static decimal? Min<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return IterateNullable (source, selector, (a, b) => a < b);
+		}
+
+		public static TResult Min<TSource, TResult> (this IEnumerable<TSource> source, Func<TSource, TResult> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			bool notAssigned = true;
+			TResult minimum = default (TResult);
+			int counter = 0;
+			foreach (TSource item in source) {
+				TResult element = selector (item);
+				if (notAssigned) {
+					minimum = element;
+					notAssigned = false;
+				} else {
+					int comparison;
+					if (element is IComparable<TResult>)
+						comparison = ((IComparable<TResult>) element).CompareTo (minimum);
+					else if (element is System.IComparable)
+						comparison = ((System.IComparable) element).CompareTo (minimum);
+					else
+						throw new ArgumentNullException ();
+
+					if (comparison < 0)
+						minimum = element;
+				}
+				counter++;
+			}
+
+			if (counter == 0)
+				throw new InvalidOperationException ();
+			else
+				return minimum;
+		}
+
+		#endregion
+
+		#region OfType
+
+		public static IEnumerable<TResult> OfType<TResult> (this IEnumerable source)
+		{
+			Check.Source (source);
+
+			return CreateOfTypeIterator<TResult> (source);
+		}
+
+		static IEnumerable<TResult> CreateOfTypeIterator<TResult> (IEnumerable source)
+		{
+			foreach (object element in source)
+				if (element is TResult)
+					yield return (TResult) element;
+		}
+
+		#endregion
+
+		#region OrderBy
+
+		public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector)
+		{
+			return OrderBy<TSource, TKey> (source, keySelector, null);
+		}
+
+		public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector,
+				IComparer<TKey> comparer)
+		{
+			Check.SourceAndKeySelector (source, keySelector);
+
+			return new OrderedSequence<TSource, TKey> (source, keySelector, comparer, SortDirection.Ascending);
+		}
+
+		#endregion
+
+		#region OrderByDescending
+
+		public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector)
+		{
+			return OrderByDescending<TSource, TKey> (source, keySelector, null);
+		}
+
+		public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
+		{
+			Check.SourceAndKeySelector (source, keySelector);
+
+			return new OrderedSequence<TSource, TKey> (source, keySelector, comparer, SortDirection.Descending);
+		}
+
+		#endregion
+
+		#region Range
+
+		public static IEnumerable<int> Range (int start, int count)
+		{
+			if (count < 0)
+				throw new ArgumentOutOfRangeException ("count");
+
+			long upto = ((long) start + count) - 1;
+
+			if (upto > int.MaxValue)
+				throw new ArgumentOutOfRangeException ();
+
+			return CreateRangeIterator (start, (int) upto);
+		}
+
+		static IEnumerable<int> CreateRangeIterator (int start, int upto)
+		{
+			for (int i = start; i <= upto; i++)
+				yield return i;
+		}
+
+		#endregion
+
+		#region Repeat
+
+		public static IEnumerable<TResult> Repeat<TResult> (TResult element, int count)
+		{
+			if (count < 0)
+				throw new ArgumentOutOfRangeException ();
+
+			return CreateRepeatIterator (element, count);
+		}
+
+		static IEnumerable<TResult> CreateRepeatIterator<TResult> (TResult element, int count)
+		{
+			for (int i = 0; i < count; i++)
+				yield return element;
+		}
+
+		#endregion
+
+		#region Reverse
+
+		public static IEnumerable<TSource> Reverse<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			var list = source as IList<TSource>;
+			if (list == null)
+				list = new List<TSource> (source);
+
+			return CreateReverseIterator (list);
+		}
+
+		static IEnumerable<TSource> CreateReverseIterator<TSource> (IList<TSource> source)
+		{
+			for (int i = source.Count; i > 0; --i)
+				yield return source [i - 1];
+		}
+
+		#endregion
+
+		#region Select
+
+		public static IEnumerable<TResult> Select<TSource, TResult> (this IEnumerable<TSource> source, Func<TSource, TResult> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return CreateSelectIterator (source, selector);
+		}
+
+		static IEnumerable<TResult> CreateSelectIterator<TSource, TResult> (IEnumerable<TSource> source, Func<TSource, TResult> selector)
+		{
+			foreach (var element in source)
+				yield return selector (element);
+		}
+
+		public static IEnumerable<TResult> Select<TSource, TResult> (this IEnumerable<TSource> source, Func<TSource, int, TResult> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return CreateSelectIterator (source, selector);
+		}
+
+		static IEnumerable<TResult> CreateSelectIterator<TSource, TResult> (IEnumerable<TSource> source, Func<TSource, int, TResult> selector)
+		{
+			int counter = 0;
+			foreach (TSource element in source) {
+				yield return selector (element, counter);
+				counter++;
+			}
+		}
+
+		#endregion
+
+		#region SelectMany
+
+		public static IEnumerable<TResult> SelectMany<TSource, TResult> (this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return CreateSelectManyIterator (source, selector);
+		}
+
+		static IEnumerable<TResult> CreateSelectManyIterator<TSource, TResult> (IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector)
+		{
+			foreach (TSource element in source)
+				foreach (TResult item in selector (element))
+					yield return item;
+		}
+
+		public static IEnumerable<TResult> SelectMany<TSource, TResult> (this IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TResult>> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return CreateSelectManyIterator (source, selector);
+		}
+
+		static IEnumerable<TResult> CreateSelectManyIterator<TSource, TResult> (IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TResult>> selector)
+		{
+			int counter = 0;
+			foreach (TSource element in source) {
+				foreach (TResult item in selector (element, counter))
+					yield return item;
+				counter++;
+			}
+		}
+
+		public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult> (this IEnumerable<TSource> source,
+			Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> selector)
+		{
+			Check.SourceAndCollectionSelectors (source, collectionSelector, selector);
+
+			return CreateSelectManyIterator (source, collectionSelector, selector);
+		}
+
+		static IEnumerable<TResult> CreateSelectManyIterator<TSource, TCollection, TResult> (IEnumerable<TSource> source,
+			Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> selector)
+		{
+			foreach (TSource element in source)
+				foreach (TCollection collection in collectionSelector (element))
+					yield return selector (element, collection);
+		}
+
+		public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult> (this IEnumerable<TSource> source,
+			Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> selector)
+		{
+			Check.SourceAndCollectionSelectors (source, collectionSelector, selector);
+
+			return CreateSelectManyIterator (source, collectionSelector, selector);
+		}
+
+		static IEnumerable<TResult> CreateSelectManyIterator<TSource, TCollection, TResult> (IEnumerable<TSource> source,
+			Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> selector)
+		{
+			int counter = 0;
+			foreach (TSource element in source)
+				foreach (TCollection collection in collectionSelector (element, counter++))
+					yield return selector (element, collection);
+		}
+
+		#endregion
+
+		#region Single
+
+		static TSource Single<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate, Fallback fallback)
+		{
+			var found = false;
+			var item = default (TSource);
+
+			foreach (var element in source) {
+				if (!predicate (element))
+					continue;
+
+				if (found)
+					throw new InvalidOperationException ();
+
+				found = true;
+				item = element;
+			}
+
+			if (!found && fallback == Fallback.Throw)
+				throw new InvalidOperationException ();
+
+			return item;
+		}
+
+		public static TSource Single<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			return source.Single (PredicateOf<TSource>.Always, Fallback.Throw);
+		}
+
+		public static TSource Single<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return source.Single (predicate, Fallback.Throw);
+		}
+
+		#endregion
+
+		#region SingleOrDefault
+
+		public static TSource SingleOrDefault<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			return source.Single (PredicateOf<TSource>.Always, Fallback.Default);
+		}
+
+		public static TSource SingleOrDefault<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return source.Single (predicate, Fallback.Default);
+		}
+
+		#endregion
+
+		#region Skip
+
+		public static IEnumerable<TSource> Skip<TSource> (this IEnumerable<TSource> source, int count)
+		{
+			Check.Source (source);
+
+			return CreateSkipIterator (source, count);
+		}
+
+		static IEnumerable<TSource> CreateSkipIterator<TSource> (IEnumerable<TSource> source, int count)
+		{
+			int i = 0;
+			foreach (var element in source) {
+				if (i++ < count)
+					continue;
+
+				yield return element;
+			}
+		}
+
+		#endregion
+
+		#region SkipWhile
+
+		public static IEnumerable<TSource> SkipWhile<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return CreateSkipWhileIterator (source, predicate);
+		}
+
+		static IEnumerable<TSource> CreateSkipWhileIterator<TSource> (IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			bool yield = false;
+
+			foreach (TSource element in source) {
+				if (yield)
+					yield return element;
+				else
+					if (!predicate (element)) {
+						yield return element;
+						yield = true;
+					}
+			}
+		}
+
+		public static IEnumerable<TSource> SkipWhile<TSource> (this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return CreateSkipWhileIterator (source, predicate);
+		}
+
+		static IEnumerable<TSource> CreateSkipWhileIterator<TSource> (IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
+		{
+			int counter = 0;
+			bool yield = false;
+
+			foreach (TSource element in source) {
+				if (yield)
+					yield return element;
+				else
+					if (!predicate (element, counter)) {
+						yield return element;
+						yield = true;
+					}
+				counter++;
+			}
+		}
+
+		#endregion
+
+		#region Sum
+
+		public static int Sum (this IEnumerable<int> source)
+		{
+			Check.Source (source);
+
+			return Sum<int, int> (source, (a, b) => checked (a + b));
+		}
+
+		public static int? Sum (this IEnumerable<int?> source)
+		{
+			Check.Source (source);
+
+			return source.SumNullable<int?, int?> (0, (total, element) => element.HasValue ? checked (total + element) : total);
+		}
+
+		public static int Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, int> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Sum<TSource, int> (source, (a, b) => checked (a + selector (b)));
+		}
+
+		public static int? Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, int?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.SumNullable<TSource, int?> (0, (a, b) => {
+				var value = selector (b);
+				return value.HasValue ? checked (a + value.Value) : a;
+			});
+		}
+
+		public static long Sum (this IEnumerable<long> source)
+		{
+			Check.Source (source);
+
+			return Sum<long, long> (source, (a, b) => checked (a + b));
+		}
+
+		public static long? Sum (this IEnumerable<long?> source)
+		{
+			Check.Source (source);
+
+			return source.SumNullable<long?, long?> (0, (total, element) => element.HasValue ? checked (total + element) : total);
+		}
+
+		public static long Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, long> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Sum<TSource, long> (source, (a, b) => checked (a + selector (b)));
+		}
+
+		public static long? Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, long?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.SumNullable<TSource, long?> (0, (a, b) => {
+				var value = selector (b);
+				return value.HasValue ? checked (a + value.Value) : a;
+			});
+		}
+
+		public static double Sum (this IEnumerable<double> source)
+		{
+			Check.Source (source);
+
+			return Sum<double, double> (source, (a, b) => checked (a + b));
+		}
+
+		public static double? Sum (this IEnumerable<double?> source)
+		{
+			Check.Source (source);
+
+			return source.SumNullable<double?, double?> (0, (total, element) => element.HasValue ? checked (total + element) : total);
+		}
+
+		public static double Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, double> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Sum<TSource, double> (source, (a, b) => checked (a + selector (b)));
+		}
+
+		public static double? Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, double?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.SumNullable<TSource, double?> (0, (a, b) => {
+				var value = selector (b);
+				return value.HasValue ? checked (a + value.Value) : a;
+			});
+		}
+
+		public static float Sum (this IEnumerable<float> source)
+		{
+			Check.Source (source);
+
+			return Sum<float, float> (source, (a, b) => checked (a + b));
+		}
+
+		public static float? Sum (this IEnumerable<float?> source)
+		{
+			Check.Source (source);
+
+			return source.SumNullable<float?, float?> (0, (total, element) => element.HasValue ? checked (total + element) : total);
+		}
+
+		public static float Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, float> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Sum<TSource, float> (source, (a, b) => checked (a + selector (b)));
+		}
+
+		public static float? Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, float?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.SumNullable<TSource, float?> (0, (a, b) => {
+				var value = selector (b);
+				return value.HasValue ? checked (a + value.Value) : a;
+			});
+		}
+
+		public static decimal Sum (this IEnumerable<decimal> source)
+		{
+			Check.Source (source);
+
+			return Sum<decimal, decimal> (source, (a, b) => checked (a + b));
+		}
+
+		public static decimal? Sum (this IEnumerable<decimal?> source)
+		{
+			Check.Source (source);
+
+			return source.SumNullable<decimal?, decimal?> (0, (total, element) => element.HasValue ? checked (total + element) : total);
+		}
+
+		public static decimal Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return Sum<TSource, decimal> (source, (a, b) => checked (a + selector (b)));
+		}
+
+		public static decimal? Sum<TSource> (this IEnumerable<TSource> source, Func<TSource, decimal?> selector)
+		{
+			Check.SourceAndSelector (source, selector);
+
+			return source.SumNullable<TSource, decimal?> (0, (a, b) => {
+				var value = selector (b);
+				return value.HasValue ? checked (a + value.Value) : a;
+			});
+		}
+
+		static TR Sum<TA, TR> (this IEnumerable<TA> source, Func<TR, TA, TR> selector)
+		{
+			TR total = default (TR);
+			long counter = 0;
+			foreach (var element in source) {
+				total = selector (total, element);
+				++counter;
+			}
+
+			return total;
+		}
+
+		static TR SumNullable<TA, TR> (this IEnumerable<TA> source, TR zero, Func<TR, TA, TR> selector)
+		{
+			TR total = zero;
+			foreach (var element in source) {
+				total = selector (total, element);
+			}
+
+			return total;
+		}
+
+		#endregion
+
+		#region Take
+
+		public static IEnumerable<TSource> Take<TSource> (this IEnumerable<TSource> source, int count)
+		{
+			Check.Source (source);
+
+			return CreateTakeIterator (source, count);
+		}
+
+		static IEnumerable<TSource> CreateTakeIterator<TSource> (IEnumerable<TSource> source, int count)
+		{
+			if (count <= 0)
+				yield break;
+
+			int counter = 0;
+			foreach (TSource element in source) {
+				yield return element;
+
+				if (++counter == count)
+					yield break;
+			}
+		}
+
+		#endregion
+
+		#region TakeWhile
+
+		public static IEnumerable<TSource> TakeWhile<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return CreateTakeWhileIterator (source, predicate);
+		}
+
+		static IEnumerable<TSource> CreateTakeWhileIterator<TSource> (IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			foreach (var element in source) {
+				if (!predicate (element))
+					yield break;
+
+				yield return element;
+			}
+		}
+
+		public static IEnumerable<TSource> TakeWhile<TSource> (this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return CreateTakeWhileIterator (source, predicate);
+		}
+
+		static IEnumerable<TSource> CreateTakeWhileIterator<TSource> (IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
+		{
+			int counter = 0;
+			foreach (var element in source) {
+				if (!predicate (element, counter))
+					yield break;
+
+				yield return element;
+				counter++;
+			}
+		}
+
+		#endregion
+
+		#region ThenBy
+
+		public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey> (this IOrderedEnumerable<TSource> source, Func<TSource, TKey> keySelector)
+		{
+			return ThenBy<TSource, TKey> (source, keySelector, null);
+		}
+
+		public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey> (this IOrderedEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
+		{
+			Check.SourceAndKeySelector (source, keySelector);
+
+			return source.CreateOrderedEnumerable (keySelector, comparer, false);
+		}
+
+		#endregion
+
+		#region ThenByDescending
+
+		public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey> (this IOrderedEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector)
+		{
+			return ThenByDescending<TSource, TKey> (source, keySelector, null);
+		}
+
+		public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey> (this IOrderedEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
+		{
+			Check.SourceAndKeySelector (source, keySelector);
+
+			return source.CreateOrderedEnumerable (keySelector, comparer, true);
+		}
+
+		#endregion
+
+		#region ToArray
+
+		public static TSource [] ToArray<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			var collection = source as ICollection<TSource>;
+			if (collection != null) {
+				var array = new TSource [collection.Count];
+				collection.CopyTo (array, 0);
+				return array;
+			}
+
+			return new List<TSource> (source).ToArray ();
+		}
+
+		#endregion
+
+		#region ToDictionary
+		public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
+		{
+			return ToDictionary<TSource, TKey, TElement> (source, keySelector, elementSelector, null);
+		}
+
+		public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
+		{
+			Check.SourceAndKeyElementSelectors (source, keySelector, elementSelector);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TKey>.Default;
+
+			var dict = new Dictionary<TKey, TElement> (comparer);
+			foreach (var e in source)
+				dict.Add (keySelector (e), elementSelector (e));
+
+			return dict;
+		}
+
+		public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector)
+		{
+			return ToDictionary (source, keySelector, null);
+		}
+
+		public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey> (this IEnumerable<TSource> source,
+				Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
+		{
+			return ToDictionary<TSource, TKey, TSource> (source, keySelector, Function<TSource>.Identity, comparer);
+		}
+
+		#endregion
+
+		#region ToList
+		public static List<TSource> ToList<TSource> (this IEnumerable<TSource> source)
+		{
+			Check.Source (source);
+
+			return new List<TSource> (source);
+		}
+		#endregion
+
+		#region ToLookup
+
+		public static ILookup<TKey, TSource> ToLookup<TSource, TKey> (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
+		{
+			return ToLookup<TSource, TKey, TSource> (source, keySelector, Function<TSource>.Identity, null);
+		}
+
+		public static ILookup<TKey, TSource> ToLookup<TSource, TKey> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
+		{
+			return ToLookup<TSource, TKey, TSource> (source, keySelector, element => element, comparer);
+		}
+
+		public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
+		{
+			return ToLookup<TSource, TKey, TElement> (source, keySelector, elementSelector, null);
+		}
+
+		public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement> (this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
+		{
+			Check.SourceAndKeyElementSelectors (source, keySelector, elementSelector);
+
+			var dictionary = new Dictionary<TKey, List<TElement>> (comparer ?? EqualityComparer<TKey>.Default);
+			foreach (var element in source) {
+				var key = keySelector (element);
+				if (key == null)
+					throw new ArgumentNullException ("key");
+
+				List<TElement> list;
+				if (!dictionary.TryGetValue (key, out list)) {
+					list = new List<TElement> ();
+					dictionary.Add (key, list);
+				}
+
+				list.Add (elementSelector (element));
+			}
+
+			return new Lookup<TKey, TElement> (dictionary);
+		}
+
+		#endregion
+
+		#region SequenceEqual
+
+		public static bool SequenceEqual<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second)
+		{
+			return first.SequenceEqual (second, null);
+		}
+
+		public static bool SequenceEqual<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+		{
+			Check.FirstAndSecond (first, second);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TSource>.Default;
+
+			using (IEnumerator<TSource> first_enumerator = first.GetEnumerator (),
+				second_enumerator = second.GetEnumerator ()) {
+
+				while (first_enumerator.MoveNext ()) {
+					if (!second_enumerator.MoveNext ())
+						return false;
+
+					if (!comparer.Equals (first_enumerator.Current, second_enumerator.Current))
+						return false;
+				}
+
+				return !second_enumerator.MoveNext ();
+			}
+		}
+
+		#endregion
+
+		#region Union
+
+		public static IEnumerable<TSource> Union<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second)
+		{
+			Check.FirstAndSecond (first, second);
+
+			return first.Union (second, null);
+		}
+
+		public static IEnumerable<TSource> Union<TSource> (this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+		{
+			Check.FirstAndSecond (first, second);
+
+			if (comparer == null)
+				comparer = EqualityComparer<TSource>.Default;
+
+			return CreateUnionIterator (first, second, comparer);
+		}
+
+		static IEnumerable<TSource> CreateUnionIterator<TSource> (IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+		{
+			var items = new HashSet<TSource> (comparer);
+			foreach (var element in first) {
+				if (! items.Contains (element)) {
+					items.Add (element);
+					yield return element;
+				}
+			}
+
+			foreach (var element in second) {
+				if (! items.Contains (element, comparer)) {
+					items.Add (element);
+					yield return element;
+				}
+			}
+		}
+
+		#endregion
+
+		#region Where
+
+		public static IEnumerable<TSource> Where<TSource> (this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return CreateWhereIterator (source, predicate);
+		}
+
+		static IEnumerable<TSource> CreateWhereIterator<TSource> (IEnumerable<TSource> source, Func<TSource, bool> predicate)
+		{
+			foreach (TSource element in source)
+				if (predicate (element))
+					yield return element;
+		}
+
+		public static IEnumerable<TSource> Where<TSource> (this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
+		{
+			Check.SourceAndPredicate (source, predicate);
+
+			return CreateWhereIterator (source, predicate);
+		}
+
+		static IEnumerable<TSource> CreateWhereIterator<TSource> (this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
+		{
+			int counter = 0;
+			foreach (TSource element in source) {
+				if (predicate (element, counter))
+					yield return element;
+				counter++;
+			}
+		}
+
+		#endregion
+
+		class ReadOnlyCollectionOf<T> {
+			public static readonly ReadOnlyCollection<T> Empty = new ReadOnlyCollection<T> (new T [0]);
+		}
+
+		internal static ReadOnlyCollection<TSource> ToReadOnlyCollection<TSource> (this IEnumerable<TSource> source)
+		{
+			if (source == null)
+				return ReadOnlyCollectionOf<TSource>.Empty;
+
+			var ro = source as ReadOnlyCollection<TSource>;
+			if (ro != null)
+				return ro;
+
+			return new ReadOnlyCollection<TSource> (source.ToArray<TSource> ());
+		}
+	}
 }