Proof using.
intuition.
destruct (held (sigma (Client c))) eqn:?; auto.
exfalso. eauto using at_head_of_nil, locks_correct_true_at_head_of_queue.
Qed.

Lemma unlock_in_flight_all_clients_false :
forall sigma p,
pBody p = Unlock ->
locks_correct_unlock sigma p ->
locks_correct sigma ->
(forall c, held (sigma (Client c)) = false).