[exim] Stumped - How do I make a router work only if a file …

OK - this doesn't work. Not sure why. Trying to make the router run a
filter file if it's there and do nothing if it's not there. The problem
is that often the parent directories aren't there either. So - how can I
make this cleanly ignore if the parent directories don't exist?

# Virtual forwarding using squirrelmail interface

#virtual_userforward:
# driver = redirect
# file = /etc/exim/control/domains/$domain/users/$local_part/filter.txt
# no_verify
# no_expn
# no_check_local_user
# modemask = 0
# check_ancestor
# user = mail
# skip_syntax_errors
# allow_filter
# allow_fail
# forbid_pipe
# forbid_filter_logwrite
# forbid_filter_lookup
# forbid_filter_readsocket
# forbid_filter_readfile
# forbid_filter_run
# file_transport = address_file
# pipe_transport = address_pipe
# reply_transport = address_reply