old_contents: class Node(object):
      def __init__(self, value, succeeding=None, previous=None):
          pass
  
  
  class LinkedList(object):
      def __init__(self):
          pass
  
new_contents: class Node(object):
      def __init__(self, value, succeeding=None, previous=None):
          self.value = value
          self.succeeding = succeeding
          self.previous = previous
  
  
  class LinkedList(object):
  
      def __init__(self):
          self.head = None
          self.tail = None
  
      def push(self, value):
          new_node = Node(value)
          if self.tail is not None:
              self.tail.succeeding = new_node
              new_node.previous = self.tail
          self.tail = new_node
  
      def pop(self):
          if self.tail is not None:
              popped_node = self.tail
              self.tail = popped_node.previous
              return popped_node.value
  
  
subject: Implement push and pop