(* OTHER FORM, w.r.t. y_1: *)

Theorem degree_L1_merge_y_1: forall(m:fmap)(x y:dart),
let m1:= L m one x y in
let y_1 := cA_1 m one y in
inv_hmap m1 -> ~expf m x y_1 ->
MF.degree m1 x = MF.degree m x + MF.degree m y_1.