static class SingleThread {
  bool running;
  
  void run(O r) { go(r); }
  
  synchronized bool go(final O runnable) {
    if (running) false;
    running = true;
    thread "Single Thread" {
      try {
        callF(runnable);
      } finally {
        _done();
      }
    }
    true;
  }
  
  synchronized void _done() {
    running = false;
  }
  
  bool running() { ret running; }
}