proposition: Lemma rev_elements_rev s : rev_elements s = rev (elements s).
proof: Proof. induction s as [|c l IHl x r IHr]; trivial. rewrite elements_node, rev_elements_node, IHl, IHr, rev_app_distr. simpl. now rewrite <- !app_assoc. Qed.
imports: Orders OrdersFacts MSetInterface PeanoNat