Section PaddX.
Variable Padd:Pol->Pol->Pol.
Variable P:Pol.
(* Xi^n * P + Q les variables de tete de Q ne sont pas forcement < i mais Q est normalisé : variables de tete decroissantes *)
Fixpoint PaddX (i n:positive)(Q:Pol){struct Q}:=
match Q with
| Pc c => mkPX P i n Q
| PX P' i' n' Q' =>
match Pos.compare i i' with
| (* i > i' *)
Gt => mkPX P i n Q
| (* i < i' *)
Lt => mkPX P' i' n' (PaddX i n Q')
| (* i = i' *)
Eq => match Z.pos_sub n n' with
| (* n > n' *)
Zpos k => mkPX (PaddX i k P') i' n' Q'
| (* n = n' *)
Z0 => mkPX (Padd P P') i n Q'
| (* n < n' *)
Zneg k => mkPX (Padd P (mkPX P' i k P0)) i n Q'
end
end
end.
End PaddX.
Fixpoint Padd (P1 P2: Pol) {struct P1} : Pol :=
match P1 with
| Pc c => PaddCl c P2
| PX P' i' n' Q' =>
PaddX Padd P' i' n' (Padd Q' P2)
end.
Notation "P ++ P'" := (Padd P P').
Definition Psub(P P':Pol):= P ++ (--P').
Notation "P -- P'" := (Psub P P')(at level 50).
(** Multiplication *)
Fixpoint PmulC_aux (P:Pol) (c:C)  {struct P} : Pol :=
match P with
| Pc c' => Pc (c' * c)
| PX P i n Q => mkPX (PmulC_aux P c) i n (PmulC_aux Q c)
end.
Definition PmulC P c :=
if c =? 0 then P0 else
if c =? 1 then P else PmulC_aux P c.
Fixpoint Pmul (P1 P2 : Pol) {struct P2} : Pol :=
match P2 with
| Pc c => PmulC P1 c
| PX P i n Q =>
PaddX Padd (Pmul P1 P) i n (Pmul P1 Q)
end.
Notation "P ** P'" := (Pmul P P')(at level 40).
Definition Psquare (P:Pol) : Pol := P ** P.
(** Evaluation of a polynomial towards R *)
Fixpoint Pphi(l:list R) (P:Pol) {struct P} : R :=
match P with
| Pc c => [c]
| PX P i n Q =>
let x := nth 0 i l in
let xn := pow_pos x n in
(Pphi l P) * xn + (Pphi l Q)
end.
Reserved Notation "P @ l " (at level 10, no associativity).
Notation "P @ l " := (Pphi l P).
(** Proofs *)
Ltac destr_pos_sub H :=
match goal with |- context [Z.pos_sub ?x ?y] =>
assert (H := Z.pos_sub_discr x y); destruct (Z.pos_sub x y)
end.

Lemma mkPX_ok : forall l P i n Q,
(mkPX P i n Q)@l == P@l * (pow_pos (nth 0 i l) n) + Q@l.