proposition: Lemma mem_Nth : forall l x, mem x l -> exists n, Nth n l x.
proof: Proof. Proof using. introv H. induction l; rew_listx in *. { inverts H. } { destruct H. { subst*. } { forwards* (n&?): IHl. } } Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)