Definition send_step (this to : nid) (d : dstatelet)
           (msg : seq nat)
           (pf : send_safe this to d msg) :=
  let C := send_safe_coh pf in 
  let q := getSt this C in
  Some (mkLocal (send_step_fun q to stag (head 0 msg))).

Lemma send_safe_def this to d msg : send_safe this to d msg <-> exists b pf, @send_step this to d msg pf = Some b.