(** Zeven, used for rounding to nearest, ties to even *)
Definition Zeven (n : Z) :=
  match n with
  | Zpos (xO _) => true
  | Zneg (xO _) => true
  | Z0 => true
  | _ => false
  end.

Theorem Zpower_plus : forall n k1 k2, (0 <= k1)%Z -> (0 <= k2)%Z -> Zpower n (k1 + k2) = (Zpower n k1 * Zpower n k2)%Z.