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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 kill_application(self, app_id, user=""):
 """Kill an application. Parameters app_id : str The id of the application to kill. user : str, optional The user to kill the application as. Requires the current user to have permissions to proxy as ``user``. Default is the current user. """

        self._call('kill', proto.KillRequest(id=app_id, user=user))