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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def stop_tracking(self, end_time = None):
 """Stop tracking current activity. end_time can be passed in if the activity should have other end time than the current moment"""

        end_time = timegm((end_time or dt.datetime.now()).timetuple())
        return self.conn.StopTracking(end_time)