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, 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 = Element(value, self.back)
  
          if not self.front:
              self.front = self.back
          else:
              self.back.ahead.behind = 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
  
  
  x = Queue()
  
  x.enqueue("A")
  print(x.front)
  print(x.back)
  print(x.front.val)
  print(x.back.val)
  x.enqueue("B")
  print(x.front)
  print(x.back)
  print(x.front.val)
  print(x.back.val)
  x.enqueue("C")
  print(x.front)
  print(x.back)
  print(x.front.val)
  print(x.back.val)
  print(x.back.ahead.val)
  print(x.front.behind.val)
  print(x.front.ahead)
  print(x.back.behind)
  print(x.back.ahead.behind.val)
  x.enqueue("D")
  print(x.front)
  print(x.back)
  print(x.front.val)
  print(x.back.val)
  print(x.back.ahead.val)
  print(x.front.behind.val)
  print(x.front.ahead)
  print(x.back.behind)
  print(x.back.ahead.behind.val)
  
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 = Element(value, self.back)
  
          if not self.front:
              self.front = self.back
          else:
              self.back.ahead.behind = 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.behind
          return val
  
      # def size(self):
      #     return size
  
  
  x = Queue()
  
  # x.enqueue("A")
  # print(x.front)
  # print(x.back)
  # print(x.front.val)
  # print(x.back.val)
  # x.enqueue("B")
  # print(x.front)
  # print(x.back)
  # print(x.front.val)
  # print(x.back.val)
  # x.enqueue("C")
  # print(x.front)
  # print(x.back)
  # print(x.front.val)
  # print(x.back.val)
  # print(x.back.ahead.val)
  # print(x.front.behind.val)
  # print(x.front.ahead)
  # print(x.back.behind)
  # print(x.back.ahead.behind.val)
  # x.enqueue("D")
  # print(x.front)
  # print(x.back)
  # print(x.front.val)
  # print(x.back.val)
  # print(x.back.ahead.val)
  # print(x.front.behind.val)
  # print(x.front.ahead)
  # print(x.back.behind)
  # print(x.back.ahead.behind.val)
  
  # x.enqueue("A")
  # x.enqueue("B")
  # x.enqueue("C")
  # x.enqueue("D")
  # print(x.dequeue())
  
  # print(x.dequeue())
  
  # print(x.dequeue())
  
  # print(x.dequeue())
  
  
  
subject: Add dequeue function