code: def make_tokens_dir(dir_, sources):
      
      os.mkdir(tokens_dir + dir_)
      for source in sources:
          if not os.path.isfile(corpora_dir + source):
              print('Invalid source: ' + source)
              return
  
      with open(tokens_dir + dir_ + '/sources.json', 'w') as outjson:
          json.dump(sources, outjson)
docs: Create a new directory named <dir_>. Create a new file within it called
      sources.json. The input <sources> is a list of names of tokenized texts.
      Write <sources> into sources.json.