def get_session_index(self):
session_index = None<EOL>authn_statement_nodes = self.__query_assertion('<STR_LIT>')<EOL>if authn_statement_nodes:<EOL><INDENT>session_index = authn_statement_nodes[<NUM_LIT:0>].get('<STR_LIT>')<EOL><DEDENT>return session_index<EOL>
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