class Queue:
# Circular List Implementation

  DEFAULT_CAPACITY = 10

  def __init__(self):
  # instance variables: _data, _size, _front

  def __str__(self):
  # string representation of queue

  def __len__(self):
  # return length of queue

  def is_empty(self):
  # return True if queue is empty otherwise return False

  def first(self):
  # return first item of queue if it is not empty; otherwise return None

  def dequeue(self):
  # remove and return first item in queue; return None if queue is empty

  def enqueue(self,e):
  # add item e to the end of queue. If list is full, resize the list by
  # creating a new list of twice the size and moving items from old list 
  # to new list; Do not forget to adjust _size and _front

  def _resize(self,cap):
  # method to resize