Proof using.
intros.
eapply message_labels_eventually_occur;
eauto using Unlock_enables_MsgUnlock, Unlock_delivered_MsgUnlock.
unfold label_silent. simpl. congruence.
Qed.

(* Specifications: *)
Lemma Nth_something_at_head :
forall A (l : list A) n x,
Nth l n x ->
exists y l',
l = y :: l'.