Proof with auto with typeclass_instances.
intros  rnd Zrnd x.
case (Req_dec x 0).
intros Zx; rewrite Zx, round_0...
unfold Rminus; rewrite Rplus_0_l, Ropp_0, Rabs_R0.
apply ulp_ge_0.
intros Zx; left.
now apply error_lt_ulp.
Qed.

(* was ulp_half_error *)
Theorem error_le_half_ulp :
forall choice x,
(Rabs (round beta fexp (Znearest choice) x - x) <= /2 * ulp x)%R.