ssh and prescript

[Jim Dishaw]

Here is the scenario (and it must be a common one).  I need to retrieve
email from a host via a SSH tunnel.  Currently I use the prescript
keyword in my mail-sources, for example

   (pop :server "localhost" :port 4747 :user "me"
        :prescript "ssh -N -n -l %u -p 443 -L %P:mailhost:110 gateway &")

Everything works--the only problem is that the SSH tunnel is not killed
and everytime GNUS retrieves mail is starts up another SSH tunnel.

Three questions:

1) Can the value for prescript be a lisp expression?  A cursory glance
   of mail-source.el, specifically mail-source-run-script, looks like it
   can be lisp expression.

2) Has anyone written such a beast?  I tried looking around, but I
   cannot find anything obvious.

3) Am I going the wrong way about this?

Thanks
-jd