(* PRESERVATION OF THE OTHER FACES IN CUTTING BY B0: OK *)

Lemma other_faces_in_cut_B0:forall (m:fmap)(x z t:dart),
inv_hmap m -> succ m zero x ->
let y := A m zero x in
let x0 := bottom m zero x in
~expf m y x0 -> ~expf m y z -> ~expf m x0 z ->
(expf m z t <-> expf (B m zero x) z t).