(* no longer useful 'cos copy is out of band and could be last *)

Lemma D_shuffle_copy :
forall q1 q2 q3 q4 : queue Message,
append Message q1 q2 = append Message q3 q4 ->
D_queue (append Message q1 (input Message copy q2)) ->
D_queue (append Message q3 (input Message copy q4)).