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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 mark_notifications(self, last_read=''):
 """Mark all notifications in this repository as read. :param str last_read: (optional), Describes the last point that notifications were checked. Anything updated since this time will not be updated. Default: Now. Expected in ISO 8601 format: ``YYYY-MM-DDTHH:MM:SSZ``. Example: "2012-10-09T23:39:01Z". :returns: bool """

        url = self._build_url('notifications', base_url=self._api)
        mark = {'read': True}
        if last_read:
            mark['last_read_at'] = last_read
        return self._boolean(self._put(url, data=dumps(mark)),
                             205, 404)