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