private Object mutex() {
    Object mutex = mutexDoNotUseDirectly;
    if (mutex == null) {
      synchronized (this) {
        mutex = mutexDoNotUseDirectly;
        if (mutex == null) {
          mutexDoNotUseDirectly = mutex = new Object();
        }
      }
    }
    return mutex;
  }

  RateLimiter(SleepingStopwatch stopwatch) {