proposition: Lemma fold_add : forall i s x, ~In x s -> eqA (fold f (add x s) i) (f x (fold f s i)).
proof: Proof. intros; apply fold_2 with (eqA := eqA); auto with set. Qed.
imports: MSetInterface PeanoNat DecidableTypeEx OrdersLists MSetFacts MSetDecide