code: def difference(self, *others):
          
          return self.copy(super(NGram, self).difference(*others))
docs: Return the difference of two or more sets as a new set.
  
          >>> from ngram import NGram
          >>> a = NGram(['spam', 'eggs'])
          >>> b = NGram(['spam', 'ham'])
          >>> list(a.difference(b))
          ['eggs']