Proof using.
unfold no_output_at_backup_trace.
intros. simpl in *. eauto.
Qed.

(* Specifications: *)
Lemma NOABT_contra :
forall A l tr,
@no_output_at_backup_trace A ((Backup, inr l) :: tr) ->
l = [] \/
exists d,
l = [ReadResponse d].