docs: def generate_highlight_subparser(subparsers):
      """Adds a sub-command parser to `subparsers` to highlight a witness'
      text with its matches in a result."""
      parser = subparsers.add_parser(
          'highlight', description=constants.HIGHLIGHT_DESCRIPTION,
          epilog=constants.HIGHLIGHT_EPILOG, formatter_class=ParagraphFormatter,
          help=constants.HIGHLIGHT_HELP)
      parser.set_defaults(func=highlight_text)
      utils.add_common_arguments(parser)
      parser.add_argument('-m', '--minus-ngrams', metavar='NGRAMS',
                          help=constants.HIGHLIGHT_MINUS_NGRAMS_HELP)
      group = parser.add_mutually_exclusive_group(required=True)
      group.add_argument('-n', '--ngrams', action='append', metavar='NGRAMS',
                         help=constants.HIGHLIGHT_NGRAMS_HELP)
      group.add_argument('-r', '--results', metavar='RESULTS',
                         help=constants.HIGHLIGHT_RESULTS_HELP)
      parser.add_argument('-l', '--label', action='append', metavar='LABEL',
                          help=constants.HIGHLIGHT_LABEL_HELP)
      utils.add_corpus_arguments(parser)
      parser.add_argument('base_name', help=constants.HIGHLIGHT_BASE_NAME_HELP,
                          metavar='BASE_NAME')
      parser.add_argument('output', metavar='OUTPUT',
                          help=constants.REPORT_OUTPUT_HELP)