import collections

class MySet(collections.MutableSet):
  __slots__=['__s']
  def __init__(self,items=None):
    if items is None:
      items=[]
    self.__s=set(items)
  def __contains__(self,v):
    return v in self.__s
  def __iter__(self):
    return iter(self.__s)
  def __len__(self):
    return len(self.__s)
  def add(self,v):
    result=v not in self.__s
    self.__s.add(v)
    return result
  def discard(self,v):
    result=v in self.__s
    self.__s.discard(v)
    return result
  def __repr__(self):
    return "MySet(%s)" % repr(list(self))


s=MySet([5,43,2,1])
print s

# default implementation of 
# pop() throws excaption here
# because of missing __next__ in set's iterator
s.pop() 
print s