[plt-scheme] fast primality testing

I'm playing around with a problem from Al Zimmerman's Programming Contest,
 http://www.recmath.org/contest/description.php
not really to try to win the contest, but just to have an excuse to
teach myself some more scheme.

One part I'm not really interested in teaching myself, though: is
there a reasonably fast primality-checker in some library somewhere?
No big requirements here: it just has to operate to return true or
false, pretty fast, for numbers up to 17 digits.

Thanks,
--Joshua Zucker