Proof with auto with typeclass_instances.
intros x Zr.
destruct (Req_dec x 0) as [Zx|Zx].
rewrite Zx, round_0...
apply ln_beta_unique.
destruct (ln_beta beta x) as (ex, Ex) ; simpl.
specialize (Ex Zx).
rewrite <- round_ZR_abs.
split.
apply round_large_pos_ge_pow...
rewrite round_ZR_abs.
now apply Rabs_pos_lt.
apply Ex.
apply Rle_lt_trans with (2 := proj2 Ex).
rewrite round_ZR_pos.
apply round_DN_pt.
apply Rabs_pos.
Qed.

Theorem ln_beta_round :
forall rnd {Hrnd : Valid_rnd rnd} x,
(round rnd x <> 0)%R ->
(ln_beta beta (round rnd x) = ln_beta beta x :> Z) \/
Rabs (round rnd x) = bpow (Zmax (ln_beta beta x) (fexp (ln_beta beta x))).