docs: def write_spelling(token_folder, spelling_file):
      """
      Generate the spelling correction file form token_folder and save to spelling_file
      """
      token_pattern = r'[a-z]{3,}'
      tokens = []
      for base, dirlist, fnlist in os.walk(token_folder):
          for fn in fnlist:
              fp = os.path.join(base, fn)
              with open(fp) as f:
                  toks = re.findall(token_pattern, f.read())
                  tokens.extend(toks)
  
      token_ranked, _ = zip(*Counter(tokens).most_common())
      with open(spelling_file, 'w') as f:
          f.write('\n'.join(token_ranked))