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