revert k n; induction s; intros; cbn.
- decide (n = k)... subst. unfold bound in H. decide (k < n0). omega. congruence.
- cbn in H. eapply Bool.andb_true_iff in H as []. erewrite IHs1, IHs2...
- erewrite IHs... omega.
Qed.

Lemma closed_app s t : closed (s t) -> closed s /\ closed t.