(** ** Power of a polynomial  - [ppower p n] is the polynomial [p] to the power [n]. *)
Fixpoint ppower (p:pol)(n:nat) : pol :=
match n with
| 0 => pcst (fst p) 1
| S n' => pmult p (ppower p n')
end.

Lemma ppower_correct p n l :
peval (ppower p n) l = power (peval p l) n.