*/
    public ClockTick clockTick(int tick) {
        lock.lock();
        try {
            return getTick(tick);
        } finally {
            lock.unlock();
        }
    }

    private ClockTickImpl getTick(int tick) {
        ClockTickImpl clockTick = ticks.get(tick);
        if (clockTick == null) {
            clockTick = new ClockTickImpl(tick);