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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 close(self):
 """The close operation loads the session if it is valid and then closes it and releases the session seat. All the session data are deleted and become invalid after the request is processed. The session ID can no longer be used in subsequent requests."""

        if self._SID:
            self._auth.service.closeSession()
            self._SID = None