(* TO BE A DOUBLE-LINK: *) 

Definition double_link(m:fmap)(x x':dart):Prop:=
  x <> x' /\ expe m x x'.

Lemma cA1_Br1:forall(m:fmap)(x x' z:dart), inv_hmap m -> cA (Br1 m x x') one z = cA m one z.