Thanks for the reply.

It's not that I want to find out which class I've got back (I'd use isa or can for that), but that I want $student->get_random_subject->get_teacher() (etc.) to give me a Lecturer for the deployment that uses Lecturers instead of Teachers, and I want to do this in the cleanest possible way.

The bit-vector idea doesn't really work in the situation that I'm working with where an arbitrary amount of tweaking can be done between deployments (some of which will be mutually incompatible).