code: def unused_by(self, bundle):
          
          if bundle is None or bundle is self.__bundle:
              # Ignore
              return
  
          with self.__usage_lock:
              try:
                  if not self.__using_bundles[bundle].dec():
                      # This bundle has cleaner all of its usages of this
                      # reference
                      del self.__using_bundles[bundle]
              except KeyError:
                  # Ignore error
                  pass
docs: Indicates that this reference is not being used anymore by the given
          bundle.
          This method should only be used by the framework.
  
          :param bundle: A bundle that used this reference