Lemma degree_L1_merge_aux: forall(m:fmap)(x y:dart)(n:nat), let m1:= L m one x y in let y0 := cA m zero y in let MAX:= MF.degree m x + MF.degree m y0 in inv_hmap m1 -> ~expf m x y0 -> 1 <= MAX - n -> MF.degree_aux m1 x (MAX - n) = MF.degree m x + MF.degree m y0.