code: def process_sentence(sentence, start_word="<S>", end_word="</S>"):
      
      if start_word is not None:
          process_sentence = [start_word]
      else:
          process_sentence = []
      process_sentence.extend(nltk.tokenize.word_tokenize(sentence.lower()))
  
      if end_word is not None:
          process_sentence.append(end_word)
      return process_sentence
docs: Seperate a sentence string into a list of string words, add start_word and end_word,
      see ``create_vocab()`` and ``tutorial_tfrecord3.py``.
  
      Parameters
      ----------
      sentence : str
          A sentence.
      start_word : str or None
          The start word. If None, no start word will be appended.
      end_word : str or None
          The end word. If None, no end word will be appended.
  
      Returns
      ---------
      list of str
          A list of strings that separated into words.
  
      Examples
      -----------
      >>> c = "how are you?"
      >>> c = tl.nlp.process_sentence(c)
      >>> print(c)
      ['<S>', 'how', 'are', 'you', '?', '</S>']
  
      Notes
      -------
      - You have to install the following package.
      - `Installing NLTK <http://www.nltk.org/install.html>`__
      - `Installing NLTK data <http://www.nltk.org/data.html>`__