sorry, now with subject... trigger & nextval(seq)

Hello all,

I'm new to triggers in PostgreSQL. I have to create a trigger on insert to
increment a sequence to overcome MS-Access' limitation in acknowledging
serial "datatype".

Could anyone put me on right track? I was looking the whole day for
references on that... Years ago I quickly found a reference how to do it
in Oracle. However, I could not find anything that explained how to do
this in postgresql... I think, it should go the direction see below... But
how exactly :-/ ?

Many thanks for any help, Torsten

create table testtab (
pid bigint,
sometext text
);

create sequence test;

-- hmm something like this...?
CREATE FUNCTION count_up (varchar) RETURNS opaque AS '
DECLARE
pid_num bigint;
BEGIN
select into pid_num from select nextval($);
RETURN pid_num;
END;
' LANGUAGE 'plpgsql';

-- and how will the trigger looks like
???