accumulate setof ?

Hi ...

I have tried to make a plgpsql function that accumulate a set of
integers, but I find no way of doing this in the PostgreSQL
documentation. I have made a very simple example to demonstrate my
problem :

CREATE FUNCTION fn_integer_list( INTEGER )
RETURNS SETOF INTEGER AS '
DECLARE
SETOF INTEGER res;
BEGIN
res := 1;
res := res + $1;
RETURN res;
END'
LANGUAGE 'plpgsql';

There is really two problems here. How does one initialize a set of
integers, and how does one append a new value to the setof integers ?
This example does, of course, not work :-)

If there is a reference to some documentation I should read, I will be
happy for any hint in its direction.

/BL