(* Idem for cA_1: *)

(* Specifications: *)
Lemma cA_1_exd :
forall (m:fmap)(k:dim)(z:dart),
inv_hmap m -> cA_1 m k z <> nil -> exd m z.
Lemma not_exd_cA_1 :
forall (m:fmap)(k:dim)(z:dart),
inv_hmap m -> ~exd m z ->
cA_1 m k z = nil.