code: def _get_words_from_dataset(dataset):
      
      # Words may be either a string or a list of tokens. Return an iterator
      # of tokens accordingly
      def tokenize(words):
          if isinstance(words, basestring):
              return word_tokenize(words, include_punc=False)
          else:
              return words
      all_words = chain.from_iterable(tokenize(words) for words, _ in dataset)
      return set(all_words)
docs: Return a set of all words in a dataset.
  
      :param dataset: A list of tuples of the form ``(words, label)`` where
          ``words`` is either a string of a list of tokens.