static IAutoCloseableF0 tempExternalMutexFor(O o) {
  Map map = externalMutexMap();
  synchronized(map) {
    O mutex = map.get(o);
    if (mutex == null)
      map.put(o, mutex = new ExternalMutex);
    mutex.referenceCount++;
    ret new IAutoCloseableF0 {
      public O get() { ret mutex; }
      public void close {
        synchronized(map) {
          if (--mutex.referenceCount == 0)
            map.remove(o);
        }
      }
    };
  }
}