<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def used_by(self, bundle):
 """ 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 """

        if bundle is None or bundle is self.__bundle:
            # Ignore
            return

        with self.__usage_lock:
            self.__using_bundles.setdefault(bundle, _UsageCounter()).inc()