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diff --git a/ag/symbol_table.c b/ag/symbol_table.c
new file mode 100755
index 0000000..b28687f
--- /dev/null
+++ b/ag/symbol_table.c
@@ -0,0 +1,152 @@
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include "symbol_table.h"
+
+struct symbol_t *new_table(void) {
+	return (struct symbol_t *)NULL;
+}
+
+struct symbol_t *clone_table(struct symbol_t *table) {
+	struct symbol_t *element;
+	struct symbol_t *new_tablex;
+
+	element=table;
+	new_tablex=new_table();
+	while((struct symbol_t *)NULL!=element) {
+		/* check return value */
+		new_tablex=table_add_symbol(new_tablex,element->identifier,element->type,0);
+		element=element->next;
+	}
+
+	return new_tablex;
+}
+
+struct symbol_t *table_add_symbol(struct symbol_t *table, char *identifier, short type, short check) {
+	struct symbol_t *element;
+	struct symbol_t *new_element;
+
+	if(table_lookup(table,identifier)!=(struct symbol_t *)NULL) {
+		if(check) {
+			fprintf(stderr,"Duplicate field %s.\n",identifier);
+			exit(3);
+		}
+
+		table=table_remove_symbol(table,identifier);
+	}
+	
+	new_element=(struct symbol_t *)malloc(sizeof(struct symbol_t));
+	new_element->next=(struct symbol_t *)NULL;
+	new_element->identifier=strdup(identifier);
+	new_element->type=type;
+
+	if((struct symbol_t *)NULL==table) {
+		return new_element;
+	}
+	element=table;
+
+	while((struct symbol_t *)NULL!=element->next) {
+		element=element->next;
+	}
+
+	element->next=new_element;
+	
+	return table;
+}
+
+struct symbol_t *table_lookup(struct symbol_t *table, char *identifier) {
+	struct symbol_t *element;
+
+	element=table;
+
+	if((struct symbol_t *)NULL==table) {
+		return (struct symbol_t *)NULL;
+	}
+	
+	if(strcmp(element->identifier,identifier)==0) {
+		return element;
+	}
+	
+	while((struct symbol_t *)NULL!=element->next) {
+		element=element->next;
+		if(strcmp(element->identifier,identifier)==0) {
+			return element;
+		}
+	}
+
+	return (struct symbol_t *)NULL;
+}
+
+struct symbol_t *table_merge(struct symbol_t *table, struct symbol_t *to_add, short check) {
+	struct symbol_t *element;
+	struct symbol_t *new_table=clone_table(table);
+	
+	element=to_add;
+	while(element!=(struct symbol_t *)NULL) {
+		new_table=table_add_symbol(new_table,element->identifier,element->type,check);
+		element=element->next;
+	}
+
+	return new_table;
+}
+
+struct symbol_t *table_remove_symbol(struct symbol_t *table, char *identifier) {
+	struct symbol_t *element;
+	struct symbol_t *previous_element;
+	struct symbol_t *new_element;
+
+	if((struct symbol_t *)NULL==table) {
+		return table;
+	}
+
+	previous_element=(struct symbol_t *)NULL;
+	element=table;
+
+	while((struct symbol_t *)NULL!=element) {
+		if(strcmp(element->identifier,identifier)==0) {
+			if((struct symbol_t *)NULL==previous_element) {
+				new_element=element->next;
+			}
+			else {
+				previous_element->next=element->next;
+				new_element=table;
+			}
+			(void)free(element->identifier);
+			(void)free(element);
+			return new_element;
+		}
+		previous_element=element;
+		element=element->next;
+	}
+
+	return table;
+}
+
+void check_variable(struct symbol_t *table, char *identifier) {
+	struct symbol_t *element=table_lookup(table,identifier);
+	if(element!=(struct symbol_t *)NULL) {
+		if(element->type!=SYMBOL_TYPE_VAR) {
+			fprintf(stderr,"Identifier %s not a variable.\n",identifier);
+			exit(3);
+		}
+	}
+	else {
+		fprintf(stderr,"Unknown identifier %s.\n",identifier);
+		exit(3);
+	}
+}
+
+void check_field(struct symbol_t *table, char *identifier) {
+	struct symbol_t *element=table_lookup(table,identifier);
+	if(element!=(struct symbol_t *)NULL) {
+		if(element->type!=SYMBOL_TYPE_FIELD) {
+			fprintf(stderr,"Identifier %s not a variable.\n",identifier);
+			exit(3);
+		}
+	}
+	else {
+		fprintf(stderr,"Unknown identifier %s.\n",identifier);
+		exit(3);
+	}
+}
+