intros b x H'; rewrite <- (Fopp_Fopp x).
apply oppBounded; auto.
Qed.

Theorem FopRepAux :
forall (b : Fbound) (z : Z) (p : R),
ex (fun r : float => r = (- p)%R :>R /\ Fbounded b r /\ Fexp r = z) ->
ex (fun r : float => r = p :>R /\ Fbounded b r /\ Fexp r = z).