intros L p H'.
apply zerop_elim_cb with (p := nf L p); auto.
Qed.
Hint Resolve zerop_nf_cb.
Definition redacc : list poly -> list poly -> list poly.
intros H'; elim H'.
intros L; exact (nil (A:=poly)).
intros a p Rec Acc.
apply LetP with (A := poly) (h := nf (p ++ Acc) a).
intros u H'0; case (zerop_dec u); intros Z.
exact (Rec Acc).
exact (u :: Rec (u :: Acc)).
Defined.
Definition red (L : list poly) : list poly := redacc L nil.
Hint Resolve incl_refl incl_tl incl_appr incl_appl incl_cons incl_app
in_or_app.

Theorem redacc_cb :
forall (L1 L2 : list poly) (p : poly),
In p (redacc L1 L2) -> cb (L1 ++ L2) p.