Proof with (try discriminate).
intros f f' EI. unfold Bexact_inverse in EI. destruct f...
destruct (Pos.eq_dec m Bexact_inverse_mantissa)...
set (e' := -e - (prec - 1) * 2) in *.
destruct (Z_le_dec emin e')...
destruct (Z_le_dec e' emax)...
inversion EI; clear EI; subst f' m.
split. auto. split. auto. split. unfold B2R. rewrite Bexact_inverse_mantissa_value.
unfold F2R; simpl. rewrite Z2R_cond_Zopp.
rewrite <- ! cond_Ropp_mult_l.
red in prec_gt_0_.
replace (Z2R (2 ^ (prec - 1))) with (bpow radix2 (prec - 1))
by (symmetry; apply (Z2R_Zpower radix2); omega).
rewrite <- ! bpow_plus.
replace (prec - 1 + e') with (- (prec - 1 + e)) by (unfold e'; omega).
rewrite bpow_opp. unfold cond_Ropp; destruct b; auto.
rewrite Ropp_inv_permute. auto. apply Rgt_not_eq. apply bpow_gt_0.
split. simpl. red; intros. apply F2R_eq_0_reg in H. destruct b; simpl in H; discriminate.
auto.
Qed.

Theorem Bdiv_mult_inverse:
forall div_nan mult_nan mode x y z,
(forall (x y z: binary_float),
is_nan _ _ x = true -> is_finite _ _ y = true -> is_finite _ _ z = true ->
div_nan x y = mult_nan x z) ->
Bexact_inverse y = Some z ->
Bdiv _ _ _ Hmax div_nan mode x y = Bmult _ _ _ Hmax mult_nan mode x z.