(***************************************************************************  Normalisation  ***************************************************************************)
Fixpoint isIn e1 p1 e2 p2 {struct e2}: option (N * PExpr C) :=
match e2 with
| e3 * e4 =>
match isIn e1 p1 e3 p2 with
| Some (N0, e5) => Some (N0, e5 ** (e4 ^^ Npos p2))
| Some (Npos p, e5) =>
match isIn e1 p e4 p2 with
| Some (n, e6) => Some (n, e5 ** e6)
| None => Some (Npos p, e5 ** (e4 ^^ Npos p2))
end
| None =>
match isIn e1 p1 e4 p2 with
| Some (n, e5) => Some (n, (e3 ^^ Npos p2) ** e5)
| None => None
end
end
| e3 ^ N0 => None
| e3 ^ Npos p3 => isIn e1 p1 e3 (Pos.mul p3 p2)
| _ => default_isIn e1 p1 e2 p2
end%poly.
Definition ZtoN z := match z with Zpos p => Npos p | _ => N0 end.
Definition NtoZ n := match n with Npos p => Zpos p | _ => Z0 end.

Lemma Z_pos_sub_gt p q : (p > q)%positive ->
Z.pos_sub p q = Zpos (p - q).