code: def GetVolumeIdentifiers(self, volume_system):
      
      volume_identifiers = []
      for volume in volume_system.volumes:
        volume_identifier = getattr(volume, 'identifier', None)
        if volume_identifier:
          volume_identifiers.append(volume_identifier)
  
      return sorted(volume_identifiers)
docs: Retrieves the volume identifiers.
  
      Args:
        volume_system (VolumeSystem): volume system.
  
      Returns:
        list[str]: sorted volume identifiers.