End POWER2.
(** Normalization and rewriting *)
Section NORM_SUBST_REC.
Let subst_l (P:Pol) := P.
Let Pmul_subst P1 P2 := subst_l (Pmul P1 P2).
Let Ppow_subst := Ppow_N subst_l.
Fixpoint norm_aux (pe:PExpr C) : Pol :=
match pe with
| PEO => Pc cO
| PEI => Pc cI
| PEc c => Pc c
| PEX _ j => mk_X j
| PEadd pe1 (PEopp pe2) =>
Psub (norm_aux pe1) (norm_aux pe2)
| PEadd pe1 pe2 => Padd (norm_aux  pe1) (norm_aux pe2)
| PEsub pe1 pe2 => Psub (norm_aux pe1) (norm_aux pe2)
| PEmul pe1 pe2 => Pmul (norm_aux pe1) (norm_aux pe2)
| PEopp pe1 => Popp (norm_aux pe1)
| PEpow pe1 n => Ppow_N (fun p => p) (norm_aux pe1) n
end.
Definition norm_subst pe := subst_l (norm_aux pe).

Lemma norm_aux_spec : forall l pe, PEeval l pe == (norm_aux pe)@l.