I'm tring to figure out how to use afew. It looks interesting, but gives me surprising results. I did `afew --learn spam -- tag:spam` and believe this tells afew that all the messages matching tag:spam should be classified as spam. But if that is right, why would `afew -c spam -- tag:spam` then show me a bunch of messages that are tagged spam by notmuch but not classified spam by afew? How do I train afew when I correct it's mistakes? Is that what `afew update` is for? What does "update the reference category" mean? Thanks much, James