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