intros p H; unfold Fdigit in |- *.
rewrite <- digitPredVNumiSPrecision.
apply digit_monotone; auto with zarith.
rewrite (fun x => Zabs_eq (Zpred x)); auto with float zarith.
Qed.

Theorem digitGivesBoundedNum :
forall p : float,
Fdigit radix p <= precision -> (Zabs (Fnum p) < Zpos (vNum b))%Z.