docs: def generate_ngrams_subparser(subparsers):
      """Adds a sub-command parser to `subparsers` to add n-grams data
      to the data store."""
      parser = subparsers.add_parser(
          'ngrams', description=constants.NGRAMS_DESCRIPTION,
          epilog=constants.NGRAMS_EPILOG, formatter_class=ParagraphFormatter,
          help=constants.NGRAMS_HELP)
      parser.set_defaults(func=generate_ngrams)
      utils.add_common_arguments(parser)
      parser.add_argument('-c', '--catalogue', dest='catalogue',
                          help=constants.NGRAMS_CATALOGUE_HELP,
                          metavar='CATALOGUE')
      utils.add_db_arguments(parser)
      utils.add_corpus_arguments(parser)
      parser.add_argument('min_size', help=constants.NGRAMS_MINIMUM_HELP,
                          metavar='MINIMUM', type=int)
      parser.add_argument('max_size', help=constants.NGRAMS_MAXIMUM_HELP,
                          metavar='MAXIMUM', type=int)