interface ISiren
{
  in void turnon();
  in void turnoff();

  behaviour c
  {
    enum States {
        Off,
        On
    };
    States state = States.Off;

    [state.Off]
    {
      on turnon:
      {
        state = States.On;
      }
      on turnoff:
        illegal;
    }
    [state.On]
    {
      on turnoff:
      {
        state = States.Off;
      }
      on turnon:
        illegal;
    }
  }
}