Trigger for grant

Below trigger will not allow to Public level grants

CREATE OR REPLACE TRIGGER ddl_trig
BEFORE GRANT ON DATABASE
DECLARE g_list dbms_standard.ora_name_list_t;
n PLS_INTEGER;
BEGIN
n := ora_grantee(g_list);
FOR i IN 1..n LOOP
IF g_list(i) = 'PUBLIC' THEN
RAISE_APPLICATION_ERROR(-20997,'Public Grants Not Allowed');
END IF;
END LOOP;
END;
/