[exim] do router based on if a files exists or not

At http://exim.org/exim-html-4.50/doc/html/spec_7.html#SECT7.3
it has this router:

   userforward:
     driver = redirect
     check_local_user
     file = $home/.forward
     no_verify
     no_expn
     check_ancestor
   # allow_filter
     file_transport = address_file
     pipe_transport = address_pipe
     reply_transport = address_reply

I'd like to implement it exactly as it *except* I only want to do it
based on if a certain file exists or not.

What would I have to add to make it only do the userforward router if -e
$home/.do_userforward ?

TIA!