def pop(self):
        """
        Pop the frame at the top of the stack.
        @return: The popped frame, else None.
        @rtype: L{Frame}
        """
        if len(self.stack):
            popped = self.stack.pop()
            log.debug('pop: (%s)\n%s', Repr(popped), Repr(self.stack))
            return popped
        else:
            log.debug('stack empty, not-popped')
        return None
Pop the frame at the top of the stack.
        @return: The popped frame, else None.
        @rtype: L{Frame}