new_contents: #!/usr/bin/env python
  # -*- coding:utf-8 -*-
  from __future__ import division
  from __future__ import unicode_literals
  from __future__ import print_function
  
  class Stack(object):
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          return self.items.pop()
  
      def top(self):
          if len(self.items) == 0:
              return None
          return self.items[-1]
  
subject: move stack.py into sequence folder