intros p H'; unfold FtoRradix, FtoR in |- *; simpl in |- *.
pattern (Fexp p) at 2 in |- *; replace (Fexp p) with (Zsucc (Zpred (Fexp p)));
[ rewrite powerRZ_Zs; auto with real zarith
| unfold Zsucc, Zpred in |- *; ring ].
repeat rewrite <- Rmult_assoc.
apply Rmult_lt_compat_r; auto with real arith.
rewrite <- Rmult_IZR; apply Rlt_IZR.
unfold pPred in |- *; cut (Zpos (vNum b) <= Zabs (Fnum p) * radix)%Z;
auto with zarith.
rewrite <- (Zabs_eq radix); auto with float zarith; rewrite <- Zabs_Zmult;
rewrite Zmult_comm; auto with float real zarith.
Qed.
Definition Fsubnormal (p : float) :=
Fbounded b p /\
Fexp p = (- dExp b)%Z /\ (Zabs (radix * Fnum p) < Zpos (vNum b))%Z.

Theorem FsubnormalFbounded : forall p : float, Fsubnormal p -> Fbounded b p.
intros p H; case H; auto.