proposition: Lemma isok_iff : forall l, Ok l <-> isok l = true.
proof: Proof. induction l. - intuition. - simpl. rewrite andb_true_iff. rewrite negb_true_iff. rewrite <- IHl. split; intros H. + inv. split; auto. apply not_true_is_false. rewrite mem_spec; auto. + destruct H; constructors; auto. rewrite <- mem_spec; auto; congruence. Qed.
imports: MSetInterface