code: def union_update(self, other):
          
          if not isinstance(other, Set):
              raise ValueError('other must be a Set instance')
          if self is other:
              return
          for item in other.items:
              self.add(item)
docs: Update the set, adding any elements from other which are not
          already in the set.
          @param other: the collection of items with which to update the set
          @type other: Set object