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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 _make_single_subvolume(self, only_one=True, **args):
 """Creates a subvolume, adds it to this class, sets the volume index to 0 and returns it. :param bool only_one: if this volume system already has at least one volume, it is returned instead. """


        if only_one and self.volumes:
            return self.volumes[0]

        if self.parent.index is None:
            index = '0'
        else:
            index = '{0}.0'.format(self.parent.index)
        volume = self._make_subvolume(index=index, **args)
        return volume