(* NEW, OK: *)

Lemma not_expf_Br1:forall (m:fmap)(x x' z t: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 z /\ ~expf m y' z
\/ ~expf m y t /\ ~expf m y' t) ->
~expf m z t -> ~expf (Br1 m x x') z t.