sclass Flag is Runnable {
  private volatile boolean up;

  /** returns true if flag was down before (i.e. flag was actually raised right now) */
  public bool raise() {
    if (up) false;
    synchronized {
      if (up) false;
      up = true;
      notifyAll();
      true;
    }
  }

  run { raise(); }
  
  public void waitUntilUp() ctex {
    if (up) ret;
    synchronized {
      while (!up) wait();
    }
  }

  public bool waitUntilUp(double timeout) {
    if (timeout == infinity()) {
      waitUntilUp();
      ret isUp();
    } else
      ret waitUntilUp(toMS(timeout));
  }
  
  public bool waitUntilUp(long timeout) ctex {
    if (up) ret true;
    synchronized {
      if (!up)
        wait(timeout);
    }
    ret isUp();
  }

  public bool isUp aka get() { ret up; }

  toString { ret isUp() ? "up" : "down"; }
}