SQL Rule

Hi list

I have a table construction like the one seen below, when i am updating
or inserting i get a recurion, logical. But how to manage it that the
rule is just doing it one time. Or is it possible to do the sum of a
and b in an other way?

CREATE TABLE test
(
a int2,
b int2,
c int2,
id int2 NOT NULL,
CONSTRAINT id_test PRIMARY KEY (id)
)
WITHOUT OIDS;
ALTER TABLE test OWNER TO postgres;

CREATE OR REPLACE RULE sum_op AS
ON INSERT TO test DO UPDATE test SET c = new.a + new.b
WHERE test.id = new.id;

CREATE OR REPLACE RULE sum_op_up AS
ON UPDATE TO test DO UPDATE test SET c = test.a + test.b
WHERE test.id = new.id;