SQL & PL/SQL :: Populate Mypo Inside Trigger?

CREATE TABLE TBL_LTEST
(
ID NUMBER,
MYPO NUMBER
);

alter table tbl_ltest add constraint pk_tbl_ltest primary key (id, mypo);

I want the data in this table to be like this:

ID MYPO
1000 401
1000 402
1000 403
2000 401
2000 402
2000 403

Rule of MYPO column population:

1. It should always start with 401 (I can handle that with Oracle check constraint that mypo should be > 401)
2. For each Id, the first insert has to be 401, then it should add one from there within the Id

(something like, mypo is a sequence within the ID.)

I am trying to populate mypo inside the trigger in this way:

CREATE OR REPLACE TRIGGER trigTBL_LTEST
BEFORE INSERT OR UPDATE
ON TBL_LTEST
REFERENCING NEW AS NEW OLD AS OLD

[code]...