def NotifyOnCancel(self, callback):
raise NotImplementedError<EOL>
Sets a callback to invoke on cancel.

        Asks that the given callback be called when the RPC is canceled.  The
        callback will always be called exactly once.  If the RPC completes without
        being canceled, the callback will be called after completion.  If the RPC
        has already been canceled when NotifyOnCancel() is called, the callback
        will be called immediately.

        NotifyOnCancel() must be called no more than once per request.