old_contents: #!/usr/bin/env python
  from __future__ import print_function
  
  
  class Element(object):
      ''' Create data element with default value and next pointer. '''
      def __init__(self, value, next=None):
          ''' Value and next pointer default to none. '''
          self.val = value
          self.next = next
  
  
  class Queue(object):
      ''' Create an empty queue. '''
      def __init__(self):
          self.front = None
          self.back = None
  
      def enqueue(self, value):
          ''' Add data element to the back of queue. '''
          self.back = Element(value, self.back)
  
          if not self.front:
              self.front = self.back
  
      def dequeue(self):
          ''' Remove front element from queue. Reassign front data
              element and return value of dequeued element. '''
          try:
              val = self.front.val
          except AttributeError:
              ''' Mimic error message from list '''
              raise IndexError("pop from empty Stack")
          self.top = self.top.previous
          return val
  
      def size(self):
          return size
  
new_contents: #!/usr/bin/env python
  from __future__ import print_function
  
  
  class Element(object):
      ''' Create data element with default value and next pointer. '''
      def __init__(self, value, ahead=None, behind=None):
          ''' Value and next pointer default to none. '''
          self.val = value
          self.ahead = ahead
          self.behind = behind
  
  
  class Queue(object):
      ''' Create an empty queue. '''
      def __init__(self):
          self.front = None
          self.back = None
  
      def enqueue(self, value):
          ''' Add data element to the back of queue. '''
          self.back.behind = Element(value, self.back)
          self.back = self.back.behind
          if not self.front:
              self.front = self.back
  
      def dequeue(self):
          ''' Remove front element from front of queue. Reassign front data
              element and return value of dequeued element. '''
          try:
              val = self.front.val
          except AttributeError:
              ''' Mimic error message from list '''
              raise IndexError("dequeue from empty Stack")
          self.front = self.front.previous
          return val
  
      def size(self):
          return size
  
subject: add behind pointer to data elements, and fix enqueue to account for behind pointer.rename next pointer to ahead pointer.