code: def _compute_frequencies(self, word_sent):
      
      freq = defaultdict(int)
      for s in word_sent:
        for word in s:
          if word not in self._stopwords:
            freq[word] += 1
      # frequencies normalization and fitering
      m = float(max(freq.values()))
      for w in freq.keys():
        freq[w] = freq[w]/m
        if freq[w] >= self._max_cut or freq[w] <= self._min_cut:
          del freq[w]
      return freq
docs: Compute the frequency of each of word.
        Input: 
         word_sent, a list of sentences already tokenized.
        Output: 
         freq, a dictionary where freq[w] is the frequency of w.