Proof with auto with typeclass_instances.
intros choice x.
destruct (generic_format_EM beta fexp x) as [Hx|Hx].
(* x = rnd x *)
rewrite round_generic...
unfold Rminus.
rewrite Rplus_opp_r, Rabs_R0.
apply Rmult_le_pos.
apply Rlt_le.
apply Rinv_0_lt_compat.
now apply (Z2R_lt 0 2).
apply ulp_ge_0.
(* x <> rnd x *)
set (d := round beta fexp Zfloor x).
destruct (round_N_pt beta fexp choice x) as (Hr1, Hr2).
destruct (Rle_or_lt (x - d) (d + ulp x - x)) as [H|H].
(* . rnd(x) = rndd(x) *)
apply Rle_trans with (Rabs (d - x)).
apply Hr2.
apply (round_DN_pt beta fexp x).
rewrite Rabs_left1.
rewrite Ropp_minus_distr.
apply Rmult_le_reg_r with 2%R.
now apply (Z2R_lt 0 2).
apply Rplus_le_reg_r with (d - x)%R.
ring_simplify.
apply Rle_trans with (1 := H).
right. field.
apply Rle_minus.
apply (round_DN_pt beta fexp x).
(* . rnd(x) = rndu(x) *)
assert (Hu: (d + ulp x)%R = round beta fexp Zceil x).
unfold d.
now rewrite <- round_UP_DN_ulp.
apply Rle_trans with (Rabs (d + ulp x - x)).
apply Hr2.
rewrite Hu.
apply (round_UP_pt beta fexp x).
rewrite Rabs_pos_eq.
apply Rmult_le_reg_r with 2%R.
now apply (Z2R_lt 0 2).
apply Rplus_le_reg_r with (- (d + ulp x - x))%R.
ring_simplify.
apply Rlt_le.
apply Rlt_le_trans with (1 := H).
right. field.
apply Rle_0_minus.
rewrite Hu.
apply (round_UP_pt beta fexp x).
Qed.

Theorem ulp_DN :
forall x,
(0 < round beta fexp Zfloor x)%R ->
ulp (round beta fexp Zfloor x) = ulp x.