(* Proof that Show for string round-trips. *)

(* Specifications: *)
Lemma not_digit_when (c : ascii) (x : nat) :
(nat_of_ascii c < 48) \/ (57 < nat_of_ascii c) ->
unit_digit x <> c.
Lemma unit_digit_ascii :
forall n, digit_of_ascii (unit_digit n) = Some (n mod 10).
Lemma decimal_thousand : forall n,
n < 256 ->
(n / 100) mod 10 * 100 + (n / 10) mod 10 * 10 + n mod 10 = n.