class ArrayStack(object):
    """ LIFO Stack implementation using a Python list as underlying storage. """
    
    def __init__(self):
        """ make an empty stack. """
        self._data = []             # non-public list instance
    
    def __len__(self):
        """Return the # of elements in the stack. """
        return len(self._data)
        
    def is_empty(self):
        """ Return True is stack is empty. """
        return len(self._data) == 0
        
    def push(self, e):
        """ Add element e to the top of the stack. """
        self._data.append(e)  # new item stored at end of list
    
    def top(self):
        """ Return (but do not remove) the element at the top of the stack.
            Raise Empty exception if the stack is empty.
        """
        if self.is_empty():
            raise ('Stack is empty!')
        return self._data[-1] # the last item in the list
        
    def pop(self):
        """ Remove and return the element from the top of the stack (i.e., LIFO)."""
        if self.is_empty():
            raise ('Stack is empty!')
        return self._data.pop()