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