(*============================================================ NEW pre_ring1 PROPERTIES: OK =============================================================*)
(* NEW, OK!! *)

(* Specifications: *)
Lemma 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 y' ->
(expf m z t -> expf (Br1 m x x') z t).