how to handle "adress already in use" after calling comm:start-up-server?

I am creating tcp servers using comm:start-up-server.  But sometimes
when I specify a port that is in use, a condition is signalled in the
thread context of the server.

How can I handle this condition, and retry on another port?  I know
how to do this when I start up another thread with
mp:process-run-function, but comm:start-up-server starts up a thread
internally, so I'm not sure how to proceed.

Thanks for any ideas...

-russ