(** 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_Zpower_nat : forall b e, (0 <= e)%Z -> Zpower b e = Zpower_nat b (Zabs_nat e).