An AF is like a virtual class in OO-speak. HyTime is a bit like a set of
virtual class definitions. From these virtual classes you derive your own
classes (DTDs) that inherit from their super-class(HyTime).
Multiple inheritance is allowwed. That is, a given DTD can inherit from
multiple AF super-classes.
I am on dodgier ground now but I think AF's are virtual rather than pure virtual
classes. I.e. an AF can be a DTD if you want it to.
Hope this helps,
Sean