proposition: Lemma mem_cons_inv_cases : forall l x y, mem x (y::l) -> x = y \/ (x <> y /\ mem x l).
proof: Proof. Proof using. introv E. rewrite* mem_cons_eq_cases in E. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)