bad variable subst after "AS"

I don't know if this is a bug or not, but if not, it looks like a misfeature ...

When executing the following in Pg 8.4.4:

CREATE OR REPLACE FUNCTION f () RETURNS TABLE(a1 INTEGER) AS
$BODY$
BEGIN
RETURN QUERY SELECT a0 AS a1 FROM rv;
RETURN;
END;
$BODY$
LANGUAGE plpgsql;

... I get this error:

ERROR: syntax error at or near "$1"
SQL state: 42601

My impression of this is that Pg is treating the "a1" after the "AS" like it was
a variable reference and so substituted it for $1.

Now that just seems wrong to me. I can understand either "a0" or "rv" getting a
substitution, but something following an "AS" being substituted is just wrong.

Is that a bug and if not then what is the rationale for working that way, and
can it be changed?

Meanwhile, what is the best way to write f to work around this misbehavior?

Thank you.

-- Darren Duncan