code: def _add_to_ngcorpus(self, corpus, words, count):
          
          if words[0] not in corpus:
              corpus[words[0]] = Counter()
  
          if len(words) == 1:
              corpus[words[0]][None] += count
          else:
              self._add_to_ngcorpus(corpus[words[0]], words[1:], count)
docs: Build up a corpus entry recursively.
  
          Parameters
          ----------
          corpus : Corpus
              The corpus
          words : [str]
              Words to add to the corpus
          count : int
              Count of words