# Stack.py
class Stack(object):

    #------------------------------------------------------------

    def __init__(self):

        '''post: creates an empty LIFO stack'''

        self.items = []

    #------------------------------------------------------------

    def push(self, item):
        
        '''post: places x on top of the stack'''

        self.items.append(item)

    #------------------------------------------------------------

    def pop(self):

        '''post: removes and returns the top element of 
        the stack'''

        return self.items.pop()

    #------------------------------------------------------------

    def top(self):

        '''post: returns the top element of the stack without 
        removing it'''

        return self.items[-1]

    #------------------------------------------------------------

    def size(self):

        '''post: returns the number of elements in the stack'''

        return len(self.items)