code: def difference(self, other):
          
  
          obj = self._clone()
          obj.difference_update(other)
          return obj
docs: Return a new set which I{self} - I{other}, i.e. the items
          in I{self} which are not also in I{other}.
  
          @param other: the other set
          @type other: Set object
          @rtype: the same type as I{self}