Proof with auto with typeclass_instances.
intros x Fx eps Heps.
case (Rle_or_lt 0 x); intros Zx.
rewrite succ_eq_pos; try assumption.
rewrite Rle_bool_true in Heps; trivial.
apply round_UP_plus_eps_pos; assumption.
(* *)
rewrite Rle_bool_false in Heps; trivial.
rewrite <- (Ropp_involutive (x+eps)).
rewrite <- (Ropp_involutive (succ x)).
rewrite round_UP_opp.
apply f_equal.
replace (-(x+eps))%R with (-succ x + (-eps + ulp (pred (-x))))%R.
apply round_DN_plus_eps_pos.
rewrite <- pred_opp.
apply pred_ge_0.
now apply Ropp_0_gt_lt_contravar.
now apply generic_format_opp.
now apply generic_format_opp, generic_format_succ.
split.
apply Rplus_le_reg_l with eps; ring_simplify.
apply Heps.
unfold pred; rewrite Ropp_involutive.
apply Rplus_lt_reg_l with (eps-ulp (- succ x))%R; ring_simplify.
apply Heps.
unfold succ; rewrite Rle_bool_false; try assumption.
apply trans_eq with (-x +-eps)%R;[idtac|ring].
pattern (-x)%R at 3; rewrite <- (pred_pos_plus_ulp (-x)).
rewrite pred_eq_pos.
ring.
left; now apply Ropp_0_gt_lt_contravar.
now apply Ropp_0_gt_lt_contravar.
now apply generic_format_opp.
Qed.

Lemma le_pred_pos_lt :
forall x y,
F x -> F y ->
(0 <= x < y)%R ->
(x <= pred_pos y)%R.