proposition: Lemma mem_cons_r : forall l x y, mem x l -> mem x (y::l).
proof: Proof. Proof using. intros. rewrite* mem_cons_eq. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)