ALTERing triggers from within a trigger

I'm trying to create the following trigger, and get compile errors on the ALTER TRIGGER lines. (Unsupported reserved word ALERT)

Does anyone know of another way to do this or what I am doing wrong here?

CREATE TRIGGER template_trigger AFTER INSERT OR DELETE on DISPLAYED_TEMPLATES FOR EACH ROW DECLARE    which_trigger VARCHAR2(38);
   which_table VARCHAR2(30);

   x                     NUMBER;

BEGIN    if DELETING then which_table := :old.table_name; end if;    if INSERTING then which_table := :new.table_name; end if;    which_trigger := which_table || '_TRIGGER';

   SELECT COUNT(*) INTO X FROM DISPLAYED_TEMPLATES WHERE table_name = which_table;

   if x = 0 then

      ALTER TRIGGER which_trigger DISABLE;    else

      ALTER TRIGGER which_trigger ENABLE;    end if;    

END template_trigger;

Thanks in advance.

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