class Stack:

  # Python list implementation of stacks

  def __init__(self):
  # construct an empty stack

  def __str__(self):
  # string representation of stack

  def __len__(self):
  # returns number of items in stack

  def is_empty(self):
  # returns True if stack is empty and False otherwise

  def top(self):
  # returns top item of stack it stack is not empty; otherwise returns None

  def pop(self):
  # similar to top, except this also deletes top item from stack.

  def push(self,e):
  # puts new item e on top of stack