Definition cn_step (this to : nid) (d : dstatelet)
           (msg : seq nat)
           (pf : cn_safe this to d msg) :=
  let C := cn_safe_coh pf in 
  let s := getStC C in
  let l := getStL C (cn_this_in (proj1 pf)) in
  Some (mkLocal (cstep_send s to (behead msg) l)).

Lemma cn_safe_def this to d msg : cn_safe this to d msg <-> exists b pf, @cn_step this to d msg pf = Some b.