(* SYMMETRICALLY: *)

Lemma cA0_1_Br1:forall(m:fmap)(x x' z:dart),
inv_hmap m -> double_link m x x' ->
cA_1 (Br1 m x x') zero z =
if eq_dart_dec (cA m zero x) z then x'
else if eq_dart_dec (cA m zero x') z then x
else cA_1 m zero z.