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Otherwise you would be able to mix both synchronous and asynchronous states in one machine, which doesn't make sense.

While tangential to my original point about naturally recursive structures, I'll take time out to disagree here too. It _can_ make sense to combine async and sync behaviour in different parts of a state machine when one is modeling electrical circuits, e.g. in certain regions of the circuit one may know that the behaviour is sync and not want to spend extra CPU by going through all the async motions.

I will quickly add that this issue (combining async and sync fsms) will occur whenever one has prior components that one wishes to assemble together, where some are async and some are sync. Dave