+(* We can now prove several interesing properties for memb:
+- memb_hd: x is a member of x::l
+- memb_cons: if x is a member of l than x is a member of a::l
+- memb_single: if x is a member of [a] then x=a
+- memb_append: if x is a member of l1@l2 then either x is a member of l1
+ or x is a member of l2
+- memb_append_l1: if x is a member of l1 then x is a member of l1@l2
+- memb_append_l2: if x is a member of l2 then x is a member of l1@l2
+- memb_exists: if x is a member of l, than l can decomposed as l1@(x::l2)
+- not_memb_to_not_eq: if x is not a member of l and y is, then x≠y
+- memb_map: if a is a member of l, then (f a) is a member of (map f l)
+- memb_compose: if a is a member of l1 and b is a meber of l2 than
+ (op a b) is a member of (compose op l1 l2)
+*)
+