intros x y H'; red in |- *.
apply Rplus_eq_reg_l with (r := (- y)%R).
repeat rewrite (Rplus_comm (- y)).
rewrite Rplus_opp_r.
change ((x - y)%R = 0%R) in |- *.
rewrite <- Fdiff_correct.
apply Rmult_eq_0_compat_r; auto.
cut (Fdiff x y = 0%Z); [ intros H; rewrite H | idtac ]; auto with real.
apply Zcompare_EGAL.
generalize H'; unfold Feq_bool, Fcompare in |- *.
case (Fdiff x y ?= 0)%Z;auto; intros; discriminate.
Qed.

Theorem Feq_bool_correct_r :
forall x y : float, Feq x y -> Feq_bool x y = true.