Reverse Scoring
===============
If you want to keep just articles that have `Sex with Emacs' in the
subject header, and expunge all other articles, you could put something
like this in your score file:
(("subject"
("Sex with Emacs" 2))
(mark 1)
(expunge 1))
So, you raise all articles that match `Sex with Emacs' and mark the
rest as read, and expunge them to boot.