[exim] Automatically filing list mail by list ID

I've just written a filter to automatically file mailing list mail in a
maildir folder based on the List-Id header sent with the message. This
seems to be working in my tests so far, but I was wondering, can anyone
see any problems with the way I've done this?

Here's the filter:

# If the message has a List-Id header which can be parsed according to
# the RFC, automatically file the message in the Lists folder
if ${address:$h_List-Id:} is not "" then
     save "$home/Maildir/.Lists.${sg{${address:$h_List-Id:}}{[./]}{-}}/"
endif

Thanks,
- Marc