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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 _AddVolume(self, volume):
 """Adds a volume. Args: volume (Volume):
 a volume. Raises: KeyError: if volume is already set for the corresponding volume identifier. """

    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)