proposition: Lemma Nth_mem : forall l x n, Nth n l x -> mem x l.
proof: Proof. Proof using. introv N. induction N; autos*. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)