sclass DoLater implements AutoCloseable {
  ISleeper sleeper;
  long targetSysTime;
  volatile Runnable action;
  transient IF1<Runnable> executor;
  bool enabled;
  
  *() {}
  *(DoLater *targetSysTime, Runnable *action) {}
  
  void enable() {
    if (enabled) ret;
    if (sleeper == null) sleeper = defaultSleeper();
    sleeper.doLater(targetSysTime, r {
      runWithExecutor(action, executor);
    });
  }
}