(* OK: USEFUL FOR THE JCT PROOF planar m USELESS, but NOT HARMFUL!! *)

Lemma not_expf_B:forall (m:fmap)(x z t:dart),
inv_hmap m -> planar m -> succ m zero x ->
let y := A m zero x in
let x0 := bottom m zero x in
(~expf m y z /\ ~expf m x0 z
\/ ~expf m y t /\ ~expf m x0 t) ->
~expf m z t -> ~expf (B m zero x) z t.