Help creating rules/triggers/functions

Hi all !

I use PostgreSQL 7.0.2 on a HP-UX system.

I would like to create a simple function and a simple trigger (or rule) that
deny a delete from a table if the row is referenced in another table.

I though it should look like this (from my Ingres experience... :) :

create function A_del(int4 i_id)
BEGIN
SELECT id
FROM b
where a_id = :i_id;

if rowcount > 0 then
RAISE EXCEPTION "not allowed !"
end if;
END

create trigger before delete from A for each row execute procedure A_del(old.id)

But it seems to be much more complicated with Postgres (create a C function
using CurrentTriggerData,...). May I have missed something or is it really much
more complicated ?

Thanks for help.

_____________
B. Carrupt