How to implement GOMONTH function

I need to create function GOMONTH which returns date by given number of
month before or forward using sql or pgsql in 8.1+
For example,
GOMONTH( DATE '20070513', 1 )  should return date '20070613'
GOMONTH( DATE '20070513', -2 )  should return date '20070313'

I tried

CREATE OR REPLACE FUNCTION public.gomonth(date, integer,
        out date) IMMUTABLE AS
$_$
SELECT $1 + $2'months';
$_$ language sql

but got error

ERROR: syntax error at or near "'months'"

How to implement this ?

Andrus.