Constraint on an aggregate? (need help writing trigger, i think..)

["Isak Hansen" <isak(dot)hansen(at)gmail(dot)com>]

I have the following two tables:

create table a (
 id serial primary key,
);
create table b (
 id serial primary key,
 a_id int4 references a (id),
 amount decimal(16, 2)
);

and would like a constraint to guarantee that "sum(b.amount) = 0 group
by b.a_id".