Mutating tables

I'm moving a system from sybase to oracle and am having some problems with triggers. I've followed the doc to set-up a trigger to set a key to -1 everytime the table is inserted. It no longer mutates but the update from the trigger does not take place. What is a good way to make this happen. Trigger follows:
create OR REPLACE trigger tI_attachee

	after insert	
 	on attachee
	FOR EACH ROW
DECLARE
	mutating_table EXCEPTION;
	PRAGMA EXCEPTION_INIT (mutating_table, -4091);

   update attachee

     set acu_stdld_key_i = -1
     where attachee.xtern_co_id_c = :old.xtern_co_id_c and 
           attachee.attachee_cd_id_c = :old.attachee_cd_id_c;
EXCEPTION
	WHEN mutating_table THEN
	null;

Thanks Received on Fri Jan 12 1996 - 00:00:00 CET