intros p q; case p; case q; unfold Fopp, Fminus, Fplus in |- *; simpl in |- *;
auto.
intros; apply floatEq; simpl in |- *; repeat rewrite (Zmin_sym Fexp0 Fexp);
repeat rewrite Zopp_mult_distr_l_reverse; auto with zarith.
Qed.

Theorem minusSameExp :
forall x y : float,
Fexp x = Fexp y -> Fminus x y = Float (Fnum x - Fnum y) (Fexp x).