docs: def generate_prepare_subparser(subparsers):
      """Adds a sub-command parser to `subparsers` to prepare source XML
      files for stripping."""
      parser = subparsers.add_parser(
          'prepare', description=constants.PREPARE_DESCRIPTION,
          epilog=constants.PREPARE_EPILOG, formatter_class=ParagraphFormatter,
          help=constants.PREPARE_HELP)
      parser.set_defaults(func=prepare_xml)
      utils.add_common_arguments(parser)
      parser.add_argument('-s', '--source', dest='source',
                          choices=constants.TEI_SOURCE_CHOICES,
                          default=constants.TEI_SOURCE_CBETA_GITHUB,
                          help=constants.PREPARE_SOURCE_HELP,
                          metavar='SOURCE')
      parser.add_argument('input', help=constants.PREPARE_INPUT_HELP,
                          metavar='INPUT')
      parser.add_argument('output', help=constants.PREPARE_OUTPUT_HELP,
                          metavar='OUTPUT')