This is one that you can actually get a lot better mileage by introducing a new state machine, oddly. You have the states A and B, but you also have the state

This is one that you can actually get a lot better mileage by introducing a new state machine, oddly. You have the states A and B, but you also have the state machine for the transition that is T1, T2, and T3. In this world, you have 5 things you need to be able to render/model. A, B, T1, T2, and T3. That the T things will be modeled/rendered inside either A or B can usually be irrelevant. Even better, if you picked the right abstraction for the T state machine, that same set of modeling can be used elsewhere. (Note, no panacea. You can still pick the wrong abstractions here. Or one that just doesn't help you later, even if it isn't wrong.) And this is exactly how it is in the physical world. The "payment acceptor" state machine of a vending machine is independent of the vending aspect of the vending machine. It is there solely to create the signal of "payment received" to the machine. Whether that comes from coins, credit cards, phone taps, whatever.