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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 get_session_index(self):
 """ Gets the SessionIndex from the AuthnStatement Could be used to be stored in the local session in order to be used in a future Logout Request that the SP could send to the SP, to set what specific session must be deleted :returns: The SessionIndex value :rtype: string|None """

        session_index = None
        authn_statement_nodes = self.__query_assertion('/saml:AuthnStatement[@SessionIndex]')
        if authn_statement_nodes:
            session_index = authn_statement_nodes[0].get('SessionIndex')
        return session_index