proposition: Lemma rev_elements_app : forall s acc, rev_elements_aux acc s = rev_elements s ++ acc.
proof: Proof. induction s; simpl; intros; auto. rewrite IHs1, IHs2. unfold rev_elements; simpl. rewrite IHs1, 2 IHs2, !app_nil_r, <- !app_assoc; auto. Qed.
imports: Orders OrdersFacts MSetInterface PeanoNat