(* cA0 / Br1 : *)

Lemma cA0_Br1:forall(m:fmap)(x x' z:dart),
inv_hmap m -> double_link m x x' ->
cA (Br1 m x x') zero z =
if eq_dart_dec x z then cA m zero x'
else if eq_dart_dec x' z then cA m zero x
else cA m zero z.