interface Timer {
    in void Create(int ms);
    in void Cancel();

    out void Timeout();

    behavior {
        bool armed = false;

        [!armed]
        {
            on Create: armed = true;
            on Cancel: {}
        }

        [armed]
        {
            on Cancel: armed = false;
            on inevitable: { armed = false; Timeout; }
        }
    }
}