How to use the new code in spam.el?

[John Wiegley]

I have the following setup: Procmail runs SpamAssassin, and then splits mail
into INBOX or mail.spam based on the headers.

In Gnus, I want to be able to mark spam in INBOX with $, and ham in mail.spam
with M-u, and have each moved to the right place.

Plus, when I leave INBOX or mail.spam, I want all the aricles passed to
sa-learn, to be learned as spam or ham, accordingly.

I'm finding the new spam.el code to be rather impenetrable.  It goes through
several layers of abstraction, such that I can't figure how to get spam/ham
learning to occur.  I see the function
`spam-spamassassin-register-spam-routine', but how do I invoke it?  What
should my `spam-process' group parameter be set to?  I have
`spam-use-spamassassin-headers' set to t before I call (spam-initialize).

Thanks,
  John