class Node:

def __init__(self, value, succeeding=None, previous=None):

self.value = value

self.succeeding = succeeding

self.prev = previous

class LinkedList:

def __init__(self):

self.head = None

self.tail = None

self.length = 0

def __len__(self):

return self.length

def __iter__(self):

current_node = self.head

while current_node:

yield (current_node.value, current_node.succeeding, current_node.prev)

current_node = current_node.succeeding

def delete(self, to_delete):

if self.length == 0:

raise ValueError("Value not found")

found = False

node = self.head

for value, succeeding, prev in self:

if value == to_delete:

if prev:

node.prev.succeeding = succeeding

else:

self.head = succeeding

if succeeding:

node.succeeding.prev = prev

else:

self.tail = prev

found = True

self.length -= 1

break

node = node.succeeding

if not found:

raise ValueError("Value not found")

def push(self, value):

new_node = Node(value)

if not self.head:

self.head = self.tail = new_node

else:

new_node.prev = self.tail

self.tail.succeeding = new_node

self.tail = new_node

self.length += 1

def pop(self):

node = self.tail

if self.length == 0:

raise IndexError("List is empty")

if node is None or node.prev is None:

self.head = self.tail = None

else:

self.tail = self.tail.prev

self.tail.succeeding = None

self.length -= 1

return node.value

def shift(self):

if self.length == 0:

raise IndexError("List is empty")

node = self.head

if node is None or node.succeeding is None:

self.head = self.tail = None

else:

self.head = self.head.succeeding

self.head.prev = None

self.length -= 1

return node.value

def unshift(self, value):

new_node = Node(value)

if not self.head:

self.head = self.tail = new_node

else:

new_node.succeeding = self.head

self.head.prev = new_node

self.head = new_node

self.length += 1