intros Y.
rewrite u_eq; unfold round.
eexists; repeat split.
simpl; now rewrite Fexp_d.
Qed.

(* Specifications: *)
Lemma m_eq: (0 < F2R d)%R ->  exists f:float beta,
F2R f = m /\ (Fexp f = fexp (ln_beta beta x) -1)%Z.