+/******************************************************************************
+bv_minus
+
+d = s1 "union" s2. ( set union operator )
+
+IN: bitvector s1 bitvector created with bv_new
+ bitvector s2 bitvector created with bv_new
+ int size Size of the bitvector
+
+IN/OUT:bitvector d bitvector created with bv_new
+
+*******************************************************************************/