(* PRESERVATION OF THE PARTITION, EXCEPT 2 MERGED FACES *)

Lemma expf_L1_x: forall (m:fmap)(x y z:dart),
let y0 := cA m zero y in
let m1 := L m one x y in
inv_hmap m1 -> ~expf m x y0 ->
expf m x z -> expf m1 x z.