docs: def add_state_machine(widget, event=None):
      """Create a new state-machine when the user clicks on the '+' next to the tabs"""
      logger.debug("Creating new state-machine...")
      root_state = HierarchyState("new root state")
      state_machine = StateMachine(root_state)
      rafcon.core.singleton.state_machine_manager.add_state_machine(state_machine)