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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def shutdown(self, callback=None):
 """Start the SSL shutdown sequence. Return a list of ssldata. The optional *callback* argument can be used to install a callback that will be called when the shutdown is complete. The callback will be called without arguments. """

        if self._state == self.S_UNWRAPPED:
            raise RuntimeError('no security layer present')
        self._state = self.S_SHUTDOWN
        self._shutdown_cb = callback
        ssldata, appdata = self.feed_ssldata(b'')
        assert appdata == [] or appdata == [b'']
        return ssldata