intros x y z n0 H' H'0 H'1 H'2; apply le_antisym.
rewrite <- H'2; auto with arith.
rewrite <- H'1; auto with arith.
Qed.

(* Specifications: *)
Theorem digit_abs : forall p : Z, digit (Zabs p) = digit p.
intros p; case p; simpl in |- *; auto.