def check_one_authnstatement(self):
authnstatement_nodes = self.__query_assertion('<STR_LIT>')<EOL>if len(authnstatement_nodes) == <NUM_LIT:1>:<EOL><INDENT>return True<EOL><DEDENT>else:<EOL><INDENT>return False<EOL><DEDENT>
Checks that the samlp:Response/saml:Assertion/saml:AuthnStatement element exists and is unique.