def pop(self):
    if self.is_empty():
        return None
    elif self._top_stack().is_empty():
        self.main_stack.pop()
        self.pop()
    return self._top_stack().pop()
def pop(self):
    if self.is_empty():
        return None
    elif self._top_stack().is_empty():
        self.main_stack.pop()
        self.pop()
    return self._top_stack().pop()