Lemma degree_L1_merge_x_others_aux: forall(m:fmap)(x y z:dart)(n:nat), let m1:= L m one x y in let y0 := cA m zero y in let dz:= MF.degree m z in inv_hmap m1 -> exd m z -> ~expf m x y0 -> ~expf m x z -> ~expf m y0 z -> n < dz -> MF.degree_aux m1 z (dz - n) = MF.degree_aux m z (dz - n).