proposition: Lemma Exists_app_l : forall P l1 l2, Exists P l1 -> Exists P (l1++l2).
proof: Proof. Proof using. intros. rewrite* Exists_app_eq. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)