what is wrong?

I am trying to use "triggers". I get following error:

create function insert_into_db2 RETURNS int AS
'/work/posgresql/lib/libpq.so' language 'C';

create trigger trial before insert or update on db1user for each row
execute procedure insert_into_db2();

ERROR: CreateTrigger: function insert_into_db2() must return OPAQUE

Does anybody know why?

Also, if a function is enforced to return OPAQUE then what is the
"RETURNS" clause?

One more question,
If a trigger does not succeed then I want to ABORT transaction, is
it possible? how?

- Sandeep