code: def summarize(self, text, n):
      
      sents = sent_tokenize(text)
      assert n <= len(sents)
      word_sent = [word_tokenize(s.lower()) for s in sents]
      self._freq = self._compute_frequencies(word_sent)
      ranking = defaultdict(int)
      for i,sent in enumerate(word_sent):
        for w in sent:
          if w in self._freq:
            ranking[i] += self._freq[w]
      sents_idx = self._rank(ranking, n)    
      return [sents[j] for j in sents_idx]
docs: Return a list of n sentences 
        which represent the summary of text.