Fixpoint Zdigits_aux (nb pow : Z) (n : nat) { struct n } : Z :=
  match n with
  | O => nb
  | S n => if Zlt_bool p pow then nb else Zdigits_aux (nb + 1) (Zmult beta pow) n
  end.(** Number of digits of an integer *)

Definition Zdigits n :=
  match n with
  | Z0 => Z0
  | Zneg p => Zdigits_aux (Zpos p) 1 beta (digits2_Pnat p)
  | Zpos p => Zdigits_aux n 1 beta (digits2_Pnat p)
  end.

Theorem Zdigits_ge_0 : forall n, (0 <= Zdigits n)%Z.