Ah, I see (sorry for my clisp inexperience):

```lisp
> (map nil #'(lambda (n) (format t "~S ~S~%" n (rationalize n))) `(3.14159265359d0  684729.48391d0 -8573.5918555d0 ,(expt 2.0d0 -100)))
3.14159265359d0 226883371/72219220
684729.48391d0 68472948391/100000
-8573.5918555d0 -13743853556/1603045
7.888609052210118d-31 1/1267650600228229260759214850049
NIL
```

I think it's a little peculiar that the exact `2.0**-100` isn't exact for `f64` (it appears to be exactly one epsilon off); I guess we don't have much choice?