Fixpoint Zgcd_pol (p : PolC Z) : (Z * Z) :=
match p with
| Pc c => (0,c)
| Pinj _ p => Zgcd_pol p
| PX p _ q =>
let (g1,c1) := Zgcd_pol p in
let (g2,c2) := Zgcd_pol q in
(ZgcdM (ZgcdM g1 c1) g2 , c2)
end.
(*Eval compute in (Zgcd_pol ((PX (Pc (-2)) 1 (Pc 4)))).*)
Fixpoint Zdiv_pol (p:PolC Z) (x:Z) : PolC Z :=
match p with
| Pc c => Pc (Z.div c x)
| Pinj j p => Pinj j (Zdiv_pol p x)
| PX p j q => PX (Zdiv_pol p x) j (Zdiv_pol q x)
end.
Inductive Zdivide_pol (x:Z): PolC Z -> Prop :=
| Zdiv_Pc : forall c, (x | c) -> Zdivide_pol x (Pc c)
| Zdiv_Pinj : forall p, Zdivide_pol x p ->  forall j, Zdivide_pol x (Pinj j p)
| Zdiv_PX   : forall p q, Zdivide_pol x p -> Zdivide_pol x q -> forall j, Zdivide_pol x (PX p j q).

Lemma Zdivide_pol_sub : forall p a b,
0 < Z.gcd a b ->
Zdivide_pol a (PsubC Z.sub p b) ->
Zdivide_pol (Z.gcd a b) p.