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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 start(self):
 """ Starts the session. Starting the session will actually get the API key of the current user """


        if NURESTSession.session_stack:
            bambou_logger.critical("Starting a session inside a with statement is not supported.")
            raise Exception("Starting a session inside a with statement is not supported.")

        NURESTSession.current_session = self

        self._authenticate()
        return self