code: def remove(self, element):
          
          if element in self._map:
              return self.evolver().remove(element).persistent()
  
          raise KeyError("Element '%s' not present in PSet" % element)
docs: Return a new PSet with element removed. Raises KeyError if element is not present.
  
          >>> s1 = s(1, 2)
          >>> s1.remove(2)
          pset([1])