How to detect nan value?

I want to process a function that returns `nan` .

I always write like

```lisp
(setq *c* (make-cube 0 0 0))
(eq (- *nan*) (send *c* :volume)) ;; t
```

However, I think it's a little weird having to write `(- *nan*)` instead of `*nan*` and ideally I want to write something like

```lisp
(if (send *c* :volume)
  ;; do something
```

Do you have any features?