def __init__(self):
    """Create an empty stack."""
    self._data = []  # nonpublic list instance

def __len__(self):
    """Return the number of elements in the stack."""
    return len(self._data)

def is_empty(self):
    """Return True if the 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 Empty('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). Raise Empty exception if the stack is empty. """
    if self.is_empty():
        raise Empty("Stack is empty")
    return self._data.pop()  # remove last item from list