I have following 2 classes node and doublylinked list

```
class DLLNode(object):
def __Init__ (self, data, prev_node=None, next_node=None):
self.data=data
self.prev_node=prev_node
self.next_node=next_node
def __str__(self):
return str(self.data)
class DoublyLinkedList(object):
def __init__(self, head=None, tail=None):
self.head=head
self.tail=tail
self.size=0
def add_to_head(self, data):
newNode = DLLNode(data)
if self.head==None:
self.head=self.tail=newNode
self.head.prev_node=self.tail.next_node=None
else:
self.head.prev_node=newNode
newNode.next_node=self.head
self.head=newNode
def add_to_tail(self, data):
newNode=DLLNode(data)
if self.head==None:
self.head=self.tail=newNode
self.head.prev_node=self.tail.next_node=None
else:
self.tail.next_node=newNode
newNode.prev_node=self.tail
self.tail=newNode
self.tail.next_node=None
def remove_head(self):
node=self.head
if self.head==self.tail:
self.prev_node=self.next_node=self.head=self.tail=None
return
if self.head!=self.tail:
node=node.next_node
node.prev_node=None
self.head=node
def remove_tail(self):
node=self.tail
if self.head==self.tail:
self.prev_node=self.next_node=self.head=self.tail=None
return
if self.head!=self.tail:
self.tail=node.prev_node
self.tail.next_node=None
def index(self,element):
current = self.head
while current != None:
if current.data == element:
return current.position
else:
current = current.next
return -1
```

I want to create a third class called SortedList, which is a subclass for DoublyLinkedList class. The class should have add and remove which add and remove object to the list, and keep the list sorted. And a 'middle' method to return the middle element of the list. Not sure how I should create this class, a little confused.