intros H e.
specialize (H (bpow (e-1))).
rewrite ulp_neq_0 in H.
2: apply Rgt_not_eq, bpow_gt_0.
unfold canonic_exp in H.
rewrite ln_beta_bpow in H.
apply generic_format_bpow_inv' in H...
now replace (e-1+1)%Z with e in H by ring.
Qed.

Theorem ulp_le_pos :
forall { Hm : Monotone_exp fexp },
forall x y: R,
(0 <= x)%R -> (x <= y)%R ->
(ulp x <= ulp y)%R.