code: def merge(self):
      with open(self.output_filename, mode="w", newline="\n") as ofile:
          for filename in glob.glob(
              self.books_path + "/" + "*.txt", recursive=True
          ):
              with open(
                  filename, mode="r", encoding="utf-8-sig", newline="\n"
              ) as file:
                  for line in file:
                      if line.strip() != "":
                          ofile.write(line.strip() + " ")
              ofile.write("\n\n")
  
file_path: gluon-nlp/src/gluonnlp/cli/data/pretrain_corpus/prepare_bookcorpus.py
imports: import os
  import tarfile
  import argparse
  import glob
  from gluonnlp.base import get_data_home_dir
  from gluonnlp.utils.misc import download, load_checksum_stats
  from collections import defaultdict
  from itertools import islice
  import time
  import multiprocessing
  import statistics
  import nltk
  import sys
  from segment_sentences import Sharding, segment_sentences, NLTKSegmenter
label_desc: Use image node_id 2 for calling the BookscorpusTextFormatting obj's underlying member method code with example usage: obj.merge() without return types