End LIST_FOLD.
(** Properties of list membership. *)

(* Specifications: *)
Lemma in_cns:
forall (A: Type) (x y: A) (l: list A), In x (y :: l) <-> y = x \/ In x l.
Lemma in_app:
forall (A: Type) (x: A) (l1 l2: list A), In x (l1 ++ l2) <-> In x l1 \/ In x l2.
Lemma list_in_insert:
forall (A: Type) (x: A) (l1 l2: list A) (y: A),
In x (l1 ++ l2) -> In x (l1 ++ y :: l2).