proposition: Lemma mem_cons_eq : forall x y l, mem x (y::l) = (x = y \/ mem x l).
proof: Proof. Proof using. intros. extens. iff H; inverts* H. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)