The I/O is done using the mutable reference to the UntypedStateMachine in the functions.

For example UntypedStateMachine could just be a vec that you append to or read from.

If you want to model cases with failure your return type will be

Result<StateMachine<Reserved>, Error>

A conditional transition should return something like

(StateMachine<PostConditional>, ConditionalData)

I think this makes sense. I’m eager to try it. Some residual questions: what is “untyped” in this case and what is “PostConditional” (an enum of possible states?)? How do subsequent transitions work?