(* OTHER FORM FOR y0: *)

Theorem degree_L1_merge_y0: forall(m:fmap)(x y:dart),
let m1:= L m one x y in
let y0 := cA m zero y in
inv_hmap m1 -> ~expf m x y0 ->
MF.degree m1 y0 = MF.degree m x + MF.degree m y0.