WithDefaultHandler.cs

    public virtual signal void start (State state = State.WORKING) {
        running = true;

        if (current_pomodoro != null) {
            current_pomodoro.end ();
        }

        current_pomodoro = new Pomodoro (state);
        seq.push_state (state);

        if (timer != null) {
            timer.cancel ();
        }

        timer = create_timer (state);
        timer.finished.connect (() => stop ());
        timer.start ();
    }

WithouDefaultHandler.cs

public signal void started (State state);

public void start (State state = State.WORKING) {
    started ();
    running = true;

    if (current_pomodoro != null) {
        current_pomodoro.end ();
    }

    current_pomodoro = new Pomodoro (state);
    seq.push_state (state);

    if (timer != null) {
        timer.cancel ();
    }

    timer = create_timer (state);
    timer.finished.connect (() => stop ());
    timer.start ();
}