Proof with auto with typeclass_instances.
intros H rndn Hrnd x Hx K.
case negligible_exp_spec'.
intros (_,Hn).
destruct (ln_beta beta x) as (e,He).
absurd (fexp e < e)%Z.
apply Zle_not_lt.
apply exp_small_round_0 with beta rndn x...
apply (Hn e).
intros (n,(H1,_)).
rewrite H in H1; discriminate.
Qed.

(* Specifications: *)
(** allows rnd x to be 0 *)
(* was ulp_error_f *)
Theorem error_lt_ulp_round :
forall { Hm : Monotone_exp fexp } rnd { Zrnd : Valid_rnd rnd } x,
( x <> 0)%R ->
(Rabs (round beta fexp rnd x - x) < ulp (round beta fexp rnd x))%R.