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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 attach_protocol(self, proto):
 """ returns a Deferred that fires once we've set this object up to track the protocol. Fails if we already have a protocol. """

        if self._protocol is not None:
            raise RuntimeError("Already have a protocol.")
        # make sure we have nothing in self.unsaved
        self.save()
        self.__dict__['_protocol'] = proto

        # FIXME some of this is duplicated from ctor
        del self.__dict__['_accept_all_']
        self.__dict__['post_bootstrap'] = defer.Deferred()
        if proto.post_bootstrap:
            proto.post_bootstrap.addCallback(self.bootstrap)
        return self.__dict__['post_bootstrap']