proposition: Lemma is_empty_spec : forall s, is_empty s = true <-> Empty s.
proof: Proof. destruct s as [|c r x l]; simpl; auto. - split; auto. intros _ x H. inv. - split; auto. + try discriminate. + intro H; elim (H x); auto. Qed.
imports: Orders OrdersFacts MSetInterface PeanoNat