The tricky thing with abstract class (oop) hierarchies is that deeply nested ones tend to be 1. hard to understand because parent behavior is shared concretely rather than modeling it behind protocols. 2. hard to change without breaking lots of dependent code because the dependencies are not obvious. 3. hard to debug because state internals easily leak out of parents and into children.

I wonder how they're managing this problem in HSM's. Seems like transitions between parent/child states would need some kind of protocol to define state entry/termination criteria so that the concrete dispatch mechanisms themselves could be abstracted away from the parents/children.

Maybe its just me.