code: def _preprocess(text, tab=4):
      
      text = re.sub(r'\r\n|\r', '\n', text)
      text = text.replace('\t', ' ' * tab)
      text = text.replace('\u00a0', ' ')
      text = text.replace('\u2424', '\n')
      pattern = re.compile(r'^ +$', re.M)
      text = pattern.sub('', text)
      text = _rstrip_lines(text)
      return text
docs: Normalize a text.