sclass RestartableCountdown {
  java.util.Timer timer;
  
  // stops the countdown and restarts it
  synchronized void start(long delayMS, O action) {
    stop();
    timer = doLater_daemon(delayMS, action);
  }
  
  void start(double delaySeconds, O action) {
    start(toMS(delaySeconds), action);
  }
  
  synchronized void stop() {
    cancelTimer(timer);
    timer = null;
  }
}