- implemented:
    abstract syntax
    - and/or states
    - pseudo-states
    - enter/exit trigger
    - shallow/deep history
    - transitions
        - event/after trigger
        - event parameters
        - guard expressions
    - action language
        expressions:
            - arithmetic, logic
            - variable references
            - function calls
        actions:
            - variable assignment
            - raising output events
    semantics
    - initialization
    - raising input events
    - time:
        - paused / step-wise
        - (scaled) real time
    - omniscient debugging


TODO

- must have:
    - explicit order of:
        - outgoing transitions
        - regions in AND-state

- usability stuff:
    - show internal events
    - highlight selected shapes while making a selection
    - comments sometimes snap to transitions even if they belong to a state
    - highlight fired transitions
    - when there is a runtime error, e.g.,
        - variable not found
        - stuck in pseudo-state
        - ???
      don't crash and show the error

- experimental features:
    - multiverse execution history
    - local scopes