intros x H' H'0; rewrite (FPredFopFSucc x).
pattern x at 1 in |- *; rewrite <- (Fopp_Fopp x).
rewrite <- Fopp_Fminus_dist.
rewrite Fopp_Fminus.
unfold FtoRradix in |- *; rewrite FSuccDiff2; auto.
rewrite <- H'; case x; auto.
Qed.

Theorem FPredDiff3 :
forall x : float,
Fnum x = nNormMin radix precision ->
Fexp x <> (- dExp b)%Z ->
Fminus radix x (FPred x) = Float 1%nat (Zpred (Fexp x)) :>R.