nodes: 0, module; 1, function_definition; 2, function_name:GetVolumeIdentifiers; 3, parameters; 4, block; 5, identifier:self; 6, identifier:volume_system; 7, expression_statement; 8, expression_statement; 9, for_statement; 10, return_statement; 11, comment:"""Retrieves the volume identifiers.
  
      Args:
        volume_system (VolumeSystem): volume system.
  
      Returns:
        list[str]: sorted volume identifiers.
      """; 12, assignment; 13, identifier:volume; 14, attribute; 15, block; 16, call; 17, identifier:volume_identifiers; 18, list; 19, identifier:volume_system; 20, identifier:volumes; 21, expression_statement; 22, if_statement; 23, identifier:sorted; 24, argument_list; 25, assignment; 26, identifier:volume_identifier; 27, block; 28, identifier:volume_identifiers; 29, identifier:volume_identifier; 30, call; 31, expression_statement; 32, identifier:getattr; 33, argument_list; 34, call; 35, identifier:volume; 36, string; 37, None; 38, attribute; 39, argument_list; 40, string_content:identifier; 41, identifier:volume_identifiers; 42, identifier:append; 43, identifier:volume_identifier
code: def GetVolumeIdentifiers(self, volume_system):
      """Retrieves the volume identifiers.
  
      Args:
        volume_system (VolumeSystem): volume system.
  
      Returns:
        list[str]: sorted volume identifiers.
      """
      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)