Proof with auto with typeclass_instances.
intros Y.
apply generic_format_bpow.
apply valid_exp.
rewrite <- Fexp_d; trivial.
apply Zlt_le_trans with  (ln_beta beta (F2R d))%Z.
rewrite Cd; apply ln_beta_generic_gt...
now apply Rgt_not_eq.
apply Hd.
apply ln_beta_le; trivial.
apply Hd.
Qed.

Lemma format_bpow_d: (0 < F2R d)%R ->
generic_format beta fexp (bpow (ln_beta beta (F2R d))).