(*=========================================================
                 CLOSURES cF, cF_1 OF F, F_1 :
==========================================================*)

(* SUCCESSOR IN A COMPLETED FACE: *)

Definition cF (m:fmap)(z:dart):=
  cA_1 m one (cA_1 m zero z).Definition cF_1 (m:fmap)(z:dart):=
  cA m zero (cA m one z).

Lemma exd_cF_not_nil : forall (m:fmap)(z:dart), inv_hmap m -> (exd m z <-> cF m z <> nil).