Proof with auto with typeclass_instances.
intros rnd Zrnd x Zx.
destruct (generic_format_EM beta fexp x) as [Hx|Hx].
(* x = rnd x *)
rewrite round_generic...
unfold Rminus.
rewrite Rplus_opp_r, Rabs_R0.
rewrite ulp_neq_0; trivial.
apply bpow_gt_0.
(* x <> rnd x *)
destruct (round_DN_or_UP beta fexp rnd x) as [H|H] ; rewrite H ; clear H.
(* . *)
rewrite Rabs_left1.
rewrite Ropp_minus_distr.
apply Rplus_lt_reg_l with (round beta fexp Zfloor x).
rewrite <- round_UP_DN_ulp with (1 := Hx).
ring_simplify.
assert (Hu: (x <= round beta fexp Zceil x)%R).
apply round_UP_pt...
destruct Hu as [Hu|Hu].
exact Hu.
elim Hx.
rewrite Hu.
apply generic_format_round...
apply Rle_minus.
apply round_DN_pt...
(* . *)
rewrite Rabs_pos_eq.
rewrite round_UP_DN_ulp with (1 := Hx).
apply Rplus_lt_reg_r with (x - ulp x)%R.
ring_simplify.
assert (Hd: (round beta fexp Zfloor x <= x)%R).
apply round_DN_pt...
destruct Hd as [Hd|Hd].
exact Hd.
elim Hx.
rewrite <- Hd.
apply generic_format_round...
apply Rle_0_minus.
apply round_UP_pt...
Qed.

(* was ulp_error_le *)
Theorem error_le_ulp :
forall rnd { Zrnd : Valid_rnd rnd } x,
(Rabs (round beta fexp rnd x - x) <= ulp x)%R.