code: def _AddVolume(self, volume):
      
      if volume.identifier in self._volumes:
        raise KeyError(
            'Volume object already set for volume identifier: {0:s}'.format(
                volume.identifier))
  
      self._volumes[volume.identifier] = volume
      self._volume_identifiers.append(volume.identifier)
docs: Adds a volume.
  
      Args:
        volume (Volume): a volume.
  
      Raises:
        KeyError: if volume is already set for the corresponding volume
            identifier.