old_contents: class ListItem(object):
      def __init__(self, data, next=None, prev=None):
          self.data = data
          self.next = next
          self.prev = prev
  
  class DoublyLinkedList(object):
      def __init__(self):
          self.head = None
          self.tail = None
          
  
new_contents: class ListItem(object):
      def __init__(self, data, next=None, prev=None):
          self.data = data
          self.next = next
          self.prev = prev
  
  class DoublyLinkedList(object):
      def __init__(self):
          self.head = None
          self.tail = None
  
      def insert(self, val):
          if self.head == None:
              self.head = self.tail = ListItem(val)
          else:
              self.head = ListItem(val, self.head)
  
subject: add insert() and pass test