(* GREAT!! *)

Theorem disconnect_planar_criterion_Br1:
forall (m:fmap)(x x':dart),
inv_hmap m -> planar m -> double_link m x x' ->
let y  := cA m zero x in
let y' := cA m zero x' in
(expf m y y' <-> ~eqc (Br1 m x x') x' y').