multiple parameters to an AGGREGATE function

Hi,
   first time poster here ...

I'm trying to write a simple Aggregate function which returns the nth
element in a collection - ultimately I want to find 95th, 90th percentiles
and so on.

It'd be called like:

select nth_element( value, 95 ) from something group by ...

I'm basing this on an example I found on the Wiki:

http://wiki.postgresql.org/wiki/Aggregate_Mode

So, I have:

CREATE OR REPLACE FUNCTION _final_nth( anyarray, integer )
  RETURNS anyelement AS
$BODY$
    SELECT a
    FROM unnest( $1 ) a
    ORDER BY a
    offset $2
    LIMIT 1;
$BODY$
LANGUAGE 'sql' IMMUTABLE;

CREATE AGGREGATE nth_element( anyelement, integer ) (
  SFUNC=array_append,
  STYPE=anyarray,
  FINALFUNC=_final_nth,
  INITCOND='{}'
);

Where the 2nd parameter would be, say 5 for the 5th element an so on.

The function declaration seems fine. But the CREATE AGGREGATE declaration
fails with:

ERROR:  function array_append(anyarray, anyelement, integer) does not exist

so, I suppose it's decided to call array_append with all the parameters on
the command line, rather than just the array in the 1st element. Is there
any way to stop it doing this?

I've searched the online documentation and Googled but haven't found
anything.

thanks,

Graham
x