intros q r H0.
apply digitInv.
replace (pred (digit q + r)) with (pred (digit q) + r).
rewrite Zpower_nat_is_exp; rewrite Zabs_Zmult;
rewrite (fun x => Zabs_eq (Zpower_nat n x)); auto with zarith arith.
generalize (digitNotZero _ H0); case (digit q); auto with arith.
intros H'; Contradict H'; auto with arith.
rewrite Zpower_nat_is_exp; rewrite Zabs_Zmult;
rewrite (fun x => Zabs_eq (Zpower_nat n x)); auto with zarith arith.
Qed.

Theorem digit_minus1 : forall p : nat, digit (Zpower_nat n p - 1) = p.
intros p; case p; auto.