rewrite <- (digit_abs radix (- Fnum x)).
rewrite <- (digit_abs radix (Fnum x)).
case (Fnum x); simpl in |- *; auto.
Qed.
Definition Fabs (x : float) := Float (Zabs (Fnum x)) (Fexp x).

Theorem Fabs_correct1 :
forall x : float, (0 <= FtoR radix x)%R -> Fabs x = x :>R.