code: def get_session_index(self):
          
          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
docs: 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