create rule and last record

i use a "create rule on insert" in order to obtain the
sum on the column montant in the column cumul.
but the last record of the table is never updated. why ?

DROP
CREATE
DROP
CREATE
CREATE 9606604 1
INSERT 9606605 1
id | montant | cumul
-----+------+-------
toto | 10 |
(1 row)

INSERT 9606606 1
id | montant | cumul
-----+------+-------
toto | 10 | 10
toto | 20 |
(2 rows)

here are the queries :

drop table test3;

create table test3 (id char(10), montant float8, cumul float8);

drop function somme(bpchar);

create function somme(bpchar) returns float8 as
'select sum(montant) from test3 where
id = \$1' language 'sql';

create rule example_rule as
on insert to test3
do
update test3 set cumul = somme(new.id);

insert into test3 (id,montant) values ('toto','10');

select * from test3;

insert into test3 (id,montant) values ('toto','20');

select * from test3;

--
Ernest CHIARELLO, chiarello(at)dr7(dot)cnrs(dot)fr, Tel:04-72-44-56-77, Fax:04-72-44-56-73
CNRS - Delegation Vallee du Rhone
2 avenue Albert Einstein 69609 VILLEURBANNE CEDEX - FRANCE