intros x H'; rewrite (FPredFopFSucc x).
pattern x at 1 in |- *; rewrite <- (Fopp_Fopp x).
rewrite <- Fopp_Fminus_dist.
rewrite Fopp_Fminus.
unfold FtoRradix in |- *; rewrite FSuccDiff1; auto.
replace (Fnum (Fopp x)) with (- Fnum x)%Z.
Contradict H'; rewrite <- (Zopp_involutive (Fnum x)); rewrite H';
auto with zarith.
case x; simpl in |- *; auto.
Qed.

Theorem FPredDiff2 :
forall x : float,
Fnum x = nNormMin radix precision ->
Fexp x = (- dExp b)%Z -> Fminus radix x (FPred x) = Float 1%nat (Fexp x) :>R.