(* AND (VERY USEFUL): *)

Theorem degree_L1_merge_x_others: forall(m:fmap)(x y z:dart),
let m1:= L m one x y in
let y0 := cA m zero y in
inv_hmap m1 -> exd m z -> ~expf m x y0 ->
~expf m x z -> ~expf m y0 z ->
MF.degree m1 z = MF.degree m z.