Definition mod2 (n : nat) : nat :=
  n - 2 * div2 n.Fixpoint power (m n:nat) : nat :=
 match n with
 | 0 => 1
 | S n' => m * power m n'
 end.

(* Specifications: *)
Lemma power_le_l : forall a b n, a <= b -> power a n <= power b n.