code: def intersection_update(self, *others):
          
          self.difference_update(super(NGram, self).difference(*others))
docs: Update the set with the intersection of itself and other sets.
  
          >>> from ngram import NGram
          >>> n = NGram(['spam', 'eggs'])
          >>> other = set(['spam', 'ham'])
          >>> n.intersection_update(other)
          >>> list(n)
          ['spam']