intros Y.
now rewrite Cd, <- ln_beta_d.
Qed.

(* Specifications: *)
Lemma format_bpow_x: (0 < F2R d)%R
-> generic_format beta fexp  (bpow (ln_beta beta x)).
Lemma format_bpow_x: (0 < F2R d)%R
-> generic_format beta fexp  (bpow (ln_beta beta x)).