proposition: Lemma mem_app : forall l1 l2 x, mem x l1 \/ mem x l2 -> mem x (l1 ++ l2).
proof: Proof. Proof using. intros. rewrite~ mem_app_eq. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)