<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def keys(self):
 """Return the list of keys in the ResultSet. :return: List of keys. Keys are tuples (series_name, tags) """

        keys = []
        for series in self._get_series():
            keys.append(
                (series.get('measurement',
                            series.get('name', 'results')),
                 series.get('tags', None))
            )
        return keys