Fixpoint norm_aux (pe:PExpr) : Pol :=
   match pe with
   | PEc c => Pc c
   | PEX j => mk_X j
   | PEadd (PEopp pe1) pe2 => Psub (norm_aux pe2) (norm_aux pe1)
   | 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).(** Internally, [norm_aux] is expanded in a large number of cases.
      To speed-up proofs, we use an alternative definition. *)

  Definition get_PEopp pe :=
   match pe with
   | PEopp pe' => Some pe'
   | _ => None
   end.

Lemma norm_aux_PEopp pe : match get_PEopp pe with | Some pe' => norm_aux pe = -- (norm_aux pe') | None => True end.