Fixpoint mkmult_rec (r:R) (lm:list (R*positive)) {struct lm}: R :=
   match lm with
   | nil => r
   | cons (x,p) t => mkmult_rec (mkmult_pow r x p) t
   end.Definition mkmult1 lm :=
   match lm with
   | nil => 1
   | cons (x,p) t => mkmult_rec (mkpow x p) t
   end.Definition mkmultm1 lm :=
   match lm with
   | nil => ropp rI
   | cons (x,p) t => mkmult_rec (mkopp_pow x p) t
   end.Definition mkmult_c_pos c lm :=
   if c ?=! cI then mkmult1 (rev' lm)
   else mkmult_rec [c] (rev' lm).Definition mkmult_c c lm :=
   match get_sign c with
   | None => mkmult_c_pos c lm
   | Some c' =>
     if c' ?=! cI then mkmultm1 (rev' lm)
     else mkmult_rec [c] (rev' lm)
   end.Definition mkadd_mult rP c lm :=
   match get_sign c with
   | None => rP + mkmult_c_pos c lm
   | Some c' => rP - mkmult_c_pos c' lm
   end.Definition add_pow_list (r:R) n l :=
   match n with
   | N0 => l
   | Npos p => (r,p)::l
   end.Fixpoint add_mult_dev
      (rP:R) (P:Pol) (fv:list R) (n:N) (lm:list (R*positive)) {struct P} : R :=
   match P with
   | Pc c =>
     let lm := add_pow_list (hd fv) n lm in
     mkadd_mult rP c lm
   | Pinj j Q =>
     add_mult_dev rP Q (jump j fv) N0 (add_pow_list (hd fv) n lm)
   | PX P i Q =>
     let rP := add_mult_dev rP P fv (N.add (Npos i) n) lm in
     if Q ?== P0 then rP
     else add_mult_dev rP Q (tail fv) N0 (add_pow_list (hd fv) n lm)
   end.Fixpoint mult_dev (P:Pol) (fv : list R) (n:N)
                     (lm:list (R*positive)) {struct P} : R :=
  (* P@l * (hd 0 l)^n * lm *)
  match P with
  | Pc c => mkmult_c c (add_pow_list (hd fv) n lm)
  | Pinj j Q => mult_dev Q (jump j fv) N0 (add_pow_list (hd fv) n lm)
  | PX P i Q =>
     let rP := mult_dev P fv (N.add (Npos i) n) lm in
     if Q ?== P0 then rP
     else
       let lmq := add_pow_list (hd fv) n lm in
       add_mult_dev rP Q (tail fv) N0 lmq
  end.Definition Pphi_avoid fv P := mult_dev P fv N0 nil.Fixpoint r_list_pow (l:list (R*positive)) : R :=
  match l with
  | nil => rI
  | cons (r,p) l => pow_pos rmul r p * r_list_pow l
  end.

Lemma Pphi_avoid_ok : forall P fv, Pphi_avoid fv P == P@fv.