code: def used_by(self, bundle):
          
          if bundle is None or bundle is self.__bundle:
              # Ignore
              return
  
          with self.__usage_lock:
              self.__using_bundles.setdefault(bundle, _UsageCounter()).inc()
docs: Indicates that this reference is being used by the given bundle.
          This method should only be used by the framework.
  
          :param bundle: A bundle using this reference