docs: def generate_diff_subparser(subparsers):
      """Adds a sub-command parser to `subparsers` to make a diff
      query."""
      parser = subparsers.add_parser(
          'diff', description=constants.DIFF_DESCRIPTION,
          epilog=constants.DIFF_EPILOG, formatter_class=ParagraphFormatter,
          help=constants.DIFF_HELP)
      parser.set_defaults(func=ngram_diff)
      group = parser.add_mutually_exclusive_group()
      group.add_argument('-a', '--asymmetric', help=constants.ASYMMETRIC_HELP,
                         metavar='LABEL')
      utils.add_common_arguments(parser)
      utils.add_db_arguments(parser)
      utils.add_corpus_arguments(parser)
      utils.add_query_arguments(parser)