Guards are boolean predicates handling the various things the machine does - follow edges, handle actions, enter and exit states, and so on - from externals, and which can prevent those transitions by returning false.

Multiple guards on the same thing will need support.

Guards are for prevention. If all you want is notification, use Hooks #255 instead.

• Entering
• Exiting
• Specific transition
• Any transition
• Standard transition
• Main transition
• Forced transition
• Data change
• Specific action
• Any action