Fixpoint Zsum_digit f k :=
match k with
| O => Z0
| S k => (Zsum_digit f k + f (Z_of_nat k) * Zpower beta (Z_of_nat k))%Z
end.

(* Specifications: *)
Theorem Zsum_digit_digit :
forall n k,
Zsum_digit (Zdigit n) k = Z.rem n (Zpower beta (Z_of_nat k)).
Theorem Zpower_gt_id :
forall n, (n < Zpower beta n)%Z.
Theorem Zdigit_ext :
forall n1 n2,
(forall k, (0 <= k)%Z -> Zdigit n1 k = Zdigit n2 k) ->
n1 = n2.
Theorem ZOmod_plus_pow_digit :
forall u v n, (0 <= u * v)%Z ->
(forall k, (0 <= k < n)%Z -> Zdigit u k = Z0 \/ Zdigit v k = Z0) ->
Z.rem (u + v) (Zpower beta n) = (Z.rem u (Zpower beta n) + Z.rem v (Zpower beta n))%Z.
Theorem ZOdiv_plus_pow_digit :
forall u v n, (0 <= u * v)%Z ->
(forall k, (0 <= k < n)%Z -> Zdigit u k = Z0 \/ Zdigit v k = Z0) ->
Z.quot (u + v) (Zpower beta n) = (Z.quot u (Zpower beta n) + Z.quot v (Zpower beta n))%Z.
Theorem Zdigit_plus :
forall u v, (0 <= u * v)%Z ->
(forall k, (0 <= k)%Z -> Zdigit u k = Z0 \/ Zdigit v k = Z0) ->
forall k,
Zdigit (u + v) k = (Zdigit u k + Zdigit v k)%Z.