import string

class UserStack:

    def __init__(self):

	self.stack = ()

    def push(self,x):

	self.stack = (x,self.stack)

    def pop(self):

	x, self.stack = self.stack
	return x

    def not_empty(self):

	return len(self.stack) != 0

    def top(self):

	return self.stack[0]

    def __len__(self):

	i = 0
	s = self.stack
	while len(s) != 0:
	    s = s[1]
	    i = i + 1
	return i

    def __repr__(self):

	l = []
	s = self.stack
	while len(s) != 0:
	    l.append(repr(s[0]))
	    s = s[1]
	return '<UserStack [%s]>' % string.join(l,', ')

    def __str__(self):

	l = []
	s = self.stack
	while len(s) != 0:
	    l.append(s[0])
	    s = s[1]
	return 's' + repr(l)