Proof with auto with typeclass_instances.
intros x Hx.
assert (Hx': (x <> 0)%R).
intros T; contradict Hx; rewrite T, Rabs_R0.
apply Rlt_not_le, bpow_gt_0.
apply Rlt_le_trans with (ulp beta fexp x)%R.
now apply error_lt_ulp...
rewrite ulp_neq_0; trivial.
unfold canonic_exp.
destruct (ln_beta beta x) as (ex, He).
simpl.
specialize (He Hx').
apply Rle_trans with (bpow (-p + 1) * bpow (ex - 1))%R.
rewrite <- bpow_plus.
apply bpow_le.
assert (emin < ex)%Z.
apply (lt_bpow beta).
apply Rle_lt_trans with (2 := proj2 He).
exact Hx.
generalize (Hmin ex).
omega.
apply Rmult_le_compat_l.
apply bpow_ge_0.
apply He.
Qed.

(** 1+#&epsilon;# property in any rounding *)
Theorem relative_error_ex :
forall x,
(bpow emin <= Rabs x)%R ->
exists eps,
(Rabs eps < bpow (-p + 1))%R /\ round beta fexp rnd x = (x * (1 + eps))%R.