code: def _make_single_subvolume(self, only_one=True, **args):
          
  
          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
docs: 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.