sclass ExternalMutex /*implements AutoCloseable*/ {
  int referenceCount;
  
  void incRefCount {
    referenceCount++;
  }
  
  int decRefCount() {
    ret --referenceCount;
  }
  
  /*public void close {
    var map = externalMutexMap();
    synchronized(map) {
      if (--referenceCount == 0)
        map.remove(this);
    }
  }*/
}