code: def union(self, *args):
          
          if self._readonly:
              raise AttributeError
  
          if len(args) == 0:
              return None
          if len(args) == 1:
              return self[args[0]]
          for a, b in zip(args[:-1], args[1:]):
              result = self._union_pair(a, b)
          return result
docs: Unions the equivalence classes containing the elements in `*args`.