proposition: Lemma app_assoc' {X: Type} {xs ys zs: list X} : (xs ++ ys) ++ zs = xs ++ ys ++ zs.
proof: Proof. by rewrite app_assoc. Qed.
imports: PeanoNat List Lia Undecidability.StackMachines.Util.Nat_facts ssreflect ssrbool ssrfun