How to enforce the use of the sequence for serial columns ?

I'd like to ensure that nobody provide the ID in an insert statement
when the id is linked to a sequence.
I tried it with a trigger, but the id value is fed before the "BEFORE
INSERT" test is performed (see below)...

Any Idea ?

Cheers,

Marc

CREATE FUNCTION serialtest() RETURNS trigger AS $serialtest$
BEGIN
-- Check that the id is provided
IF NEW.id IS NOT NULL THEN
RAISE EXCEPTION 'id will be set from a sequence; do not
provide it!';
END IF;

RETURN NEW;
END;
$serialtest$ LANGUAGE plpgsql;

CREATE TABLE test_table
(
id serial primary key,
foo int
);

CREATE TRIGGER test BEFORE INSERT OR UPDATE ON test_table
FOR EACH ROW EXECUTE PROCEDURE serialtest();

insert into test_table(foo)values(1);

ERROR: id will be set from a sequence; do not provide it!
SQL state: P0001