proposition: Lemma app_cancel_l : forall l1 l2 l3, l1 ++ l2 = l1 ++ l3 -> l2 = l3.
proof: Proof. Proof using. introv E. induction l1; rew_list in *. auto. inverts* E. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)