intros f1 f2 H' H'0 H'1 H'2; red in |- *.
rewrite
FcanonicUnique
with
(3 := pGivesBound)
(p := Fnormalize radix b precision f2)
(q := Fnormalize radix b precision f1);
auto with float arith.
repeat rewrite FnormalizeCorrect; auto.
Qed.

Theorem FNevenFop : forall p : float, FNeven p -> FNeven (Fopp p).
intros p; unfold FNeven in |- *.