Definition Zdigits2 n :=
  match n with
  | Z0 => n
  | Zpos p => Zpos (digits2_pos p)
  | Zneg p => Zpos (digits2_pos p)
  end.

(* Specifications: *)
Lemma Zdigits2_Zdigits : forall n, Zdigits2 n = Zdigits radix2 n.