Proof with auto with typeclass_instances.
intros x y Hx Hy Zx H.
rewrite succ_eq_pos; trivial.
case (Rle_or_lt (ulp x) (y-x)); intros H1.
apply Rplus_le_reg_r with (-x)%R.
now ring_simplify (x+ulp x + -x)%R.
replace y with (x+(y-x))%R by ring.
absurd (x < y)%R.
2: apply H.
apply Rle_not_lt; apply Req_le.
rewrite <- round_DN_plus_eps_pos with (eps:=(y-x)%R); try easy.
ring_simplify (x+(y-x))%R.
apply sym_eq.
apply round_generic...
split; trivial.
apply Rlt_le; now apply Rlt_Rminus.
Qed.

Theorem succ_le_lt:
forall x y,
F x -> F y ->
(x < y)%R ->
(succ x <= y)%R.