Definition is_nan f :=
  match f with
  | B754_nan _ _ => true
  | _ => false
  end.Definition is_nan_FF f :=
  match f with
  | F754_nan _ _ => true
  | _ => false
  end.

Theorem is_nan_FF2B : forall x Hx, is_nan (FF2B x Hx) = is_nan_FF x.