proposition: Lemma Nth_app_l : forall n x l1 l2, Nth n l1 x -> Nth n (l1 ++ l2) x.
proof: Proof. Proof using. intros n. induction n; introv H; inverts H; rew_list*. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)