sclass DoLater implements AutoCloseable {
  long targetSysTime;
  volatile Runnable action;
  transient settable ISleeper sleeper;
  transient Executor executor;
  transient new Flag enabled;
  transient new Flag cancelledOrExecuted;
  
  *() {}
  *(long *targetSysTime, Runnable *action) {}
  
  void enable() {
    if (!enabled.raise()) ret;
    if (sleeper == null) sleeper = defaultSleeper();
    sleeper.doLater(targetSysTime, r {
      if (!cancelledOrExecuted.raise()) ret;
      runWithExecutor(action, executor);
    });
  }
  
  // returns true iff successful
  bool cancel() {
    if (!cancelledOrExecuted.raise()) false;
    ret true with close();
  }
  
  public void close {
    cancelledOrExecuted.raise();
    //dispose sleeper;
  }
}