code: def difference_update(self, other):
          
          if not isinstance(other, Set):
              raise ValueError('other must be a Set instance')
          if self is other:
              self.items = []
          else:
              for item in other.items:
                  self.discard(item)
docs: Update the set, removing any elements from other which are in
          the set.
          @param other: the collection of items with which to update the set
          @type other: Set object