new_contents: #!/usr/bin/python -e
  # -*- encoding: utf-8 -*-
  #
  # Copyright (c) 2013 ASMlover. All rights reserved.
  #
  # Redistribution and use in source and binary forms, with or without
  # modification, are permitted provided that the following conditions
  # are met:
  #
  #  * Redistributions of source code must retain the above copyright
  #    notice, this list ofconditions and the following disclaimer.
  #
  #  * Redistributions in binary form must reproduce the above copyright
  #    notice, this list of conditions and the following disclaimer in
  #    the documentation and/or other materialsprovided with the
  #    distribution.
  #
  # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  # FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
  # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
  # BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  # CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
  # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  # POSSIBILITY OF SUCH DAMAGE.
  
  class Stack(object):
    """ Stack of mini-stl"""
    class StackNode(object):
      def __init__(self):
        self.next = None
        self.data = None 
      def __del__(self):
        self.next = None
        self.data = None 
    
    def __init__(self):
      self.head_ = None 
    
    def __del__(self):
      self.clear() 
    
    def clear(self):
      while self.head_ != None:
        node = self.head_
        self.head_ = self.head_.next
        del node 
    
    def empty(self):
      return self.head_ == None
  
    def push(self, x):
      node = self.StackNode()
      node.next = self.head_
      node.data = x
      self.head_ = node 
  
    def pop(self): 
      assert self.head_ != None 
      node = self.head_
      self.head_ = self.head_.next 
      del node  
    
    def top(self):
      assert self.head_ != None 
      return self.head_.data
  
  
subject: add stack module for mini-stl(Python) use single list