<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_subscribers(self, subscribers, **kwargs):
 """ Add a callbacks to be invoked during transfer """

        if subscribers:
            with self._callbacks_lock:
                self._add_subscribers_for_type(
                    'done', subscribers, self._done_callbacks, **kwargs)
                self._add_subscribers_for_type(
                    'queued', subscribers, self._queued_callbacks, **kwargs)
                self._add_subscribers_for_type(
                    'progress', subscribers, self._progress_callbacks, **kwargs)