code: def update(self, iterable):
          
          e = self.evolver()
          for element in iterable:
              e.add(element)
  
          return e.persistent()
docs: Return a new PSet with elements in iterable added
  
          >>> s1 = s(1, 2)
          >>> s1.update([3, 4, 4])
          pset([1, 2, 3, 4])