problem with automatic altering of groups

Hello, 
is ist possible to write a function that alters the group of a given user like the following should do in my 
opinion (I'd like to use it in a rule): 
CREATE FUNCTION set_group (text, integer) RETURNS text AS ' ALTER GROUP userlevel1 DROP USER $1; ALTER GROUP userlevel2
DROPUSER $1; ALTER GROUP 
 
userlevel3 DROP USER $1; ALTER GROUP userlevel4 DROP USER $1; ALTER GROUP userlevel5 DROP USER $1; CASE $2     WHEN 5
THEN        (ALTER GROUP userlevel5 ADD USER $1)     WHEN 4 THEN         (ALTER GROUP userlevel5 ADD USER $1; ALTER
GROUPuserlevel4 ADD USER $1;)     [...] END; SELECT ''; 
 
' LANGUAGE 'SQL'; 
I don't understand why this function ist refused by postgresql with an parse error at the first $1 as ist every 
other method I tryed to get postgres making an "alter group" with a variable. I'd be very happy if someone 
could help... 
yours 
Matthias Nagl 
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