DATABASE TRIGGERS

I have a question, base on ORACLE MANUAL say a trigger can not refer to itself (mutating table) and with this example :

create table ab (colu char (10));  

create or replace trigger ab_before before insert on ab for each row declare
wk char(10);
begin
update ab set colu = 'DD' where rownum =1; end;

insert into ab values ('ABCD');

this should raise the exception :
ORA-04091: table PCHIEN.AA is mutating, trigger may not read or modify it

but I can successful insert the row.

can somebody tell me why ???

thankx.

--
Chao-Ping Chien,    DBA
Trimark Inv. Mgmt Inc.
Canada
pchien_at_trimark.com
pchien_at_tor.hookup.net