Dealing with AnObsoleteC3 Class

I'm trying to refactor a piece of code using the refactoring browser,
but I've started getting messages telling me that AnObsoleteC3 class
does not understand #directlyDefinesMethod.

I assume that a changeset I've loaded has removed a class called C3,
but that there's still some reference to it hanging around. I guess I
need help with two things:
- how do I track this thing down?
- how do I kill it?

I've inspected 'self' from the debugger, and had a poke around some of
the pointer tools, but don't really know what I'm doing. Can anyone
enlighten me?