20141125a

s = Stack()     # create an empty stack
print s         # [
print s.empty() # True
s.push(1)
s.push(4)
s.push(5)
print s         # [1 4 5
print s.size()  # 3
print s.empty() # False
x = s.pop()
print x         # 5
print s         # [1 4
s.pop()
s.pop()         # now it's empty
x = s.pop()
print x         # None (means that we wanted to take an element out of an empty stack)