function error

CREATE FUNCTION biwklyavg(text, text, text) RETURNS numeric AS '
DECLARE
system ALIAS $1;
startdate ALIAS $2;
enddate ALIAS $3;
result numeric;

BEGIN
result := (select (avg(usr) + avg(sys)) from sardata where
systemid = system
and time between startdate and enddate;
END;
' LANGUAGE 'sql';

Hello,

The fuction listed above gives the following error:

ERROR: parser: parse error at or near "alias"

I'm running v7.2. Any ideas? I pulled this directly from the examples in
the PostgreSQL Developer's Handbook.

Thanks,

Travis