(* j-SUCCS OF y_1 BY cF_L1, FOR j < DEG OF y0 (OR y_1), ARE INVARIANT:*)

Lemma cF_L1_y_1_y0:forall(m:fmap)(x y:dart)(j:nat),
inv_hmap m -> prec_L m one x y ->
let m1:= L m one x y in
let y0 := cA m zero y in
let y_1 := cA_1 m one y in
let dx := MF.degree m x in
let dy0 := MF.degree m y0 in
~expf m x y0 ->
j <= dy0 -1 ->
Iter (cF m1) (dx + j) x = Iter (cF m) j y_1.