calling a function that takes a row type and returns a set of rows

So, say I have something like this - the actual example is something a
bit more useful:

CREATE TABLE foo (a integer, b integer);
INSERT INTO foo VALUES (1, 1); -- must have some data to generate the failure

CREATE FUNCTION bar (foo) RETURNS SETOF foo AS $$
DECLARE
f foo;
BEGIN
f.a := 1;
RETURN NEXT f;
f.a := 2;
RETURN NEXT f;
END
$$ LANGUAGE plpgsql;

I can't find any legal way of calling this function.

SELECT bar(f) FROM foo f;
ERROR: set-valued function called in context that cannot accept a set

SELECT * FROM foo f, bar(f);
ERROR: function expression in FROM may not refer to other relations
of same query level

Any help appreciated.

Thanks,

...Robert