replace 0%Z with (- 0%nat)%Z; [ idtac | simpl in |- *; auto ].
apply Zeq_le; auto with arith.
Qed.
Hint Unfold Fbounded.

Theorem FboundedZeroSameExp :
forall (b : Fbound) (p : float), Fbounded b p -> Fbounded b (Fzero (Fexp p)).