class Stack:

def __init__(self, elem = None, lst = None):

if lst is not None:

self.stack = Create_from_list(elem, lst)

else:

self.stack = Create_list([])

def push(self, elem):

return Stack(elem, self.stack)

def pop(self):

if self.stack.tail is not Nil:

return Stack(nth(1, self.stack), lself.stack.tail.tail)

else:

return Nil()

def is_empty(self):

return empty(self.stack)

def peek(self):

return nth(0, self.stack)

def __str__(self):

if self.stack is Nil:

return '()'

else:

return self.stack.__str__()

'''

s1 = Stack()

s2 = s1.push(1)

print(s2.is_empty())

print(s2)

outcome ->

False

(1)

'''