naming arguments in aggregate function

Hi,
I am trying to name arguments in aggregate function, but i don't know how,
so im asking if someone can help me to solve this.
Im working with 8.3.11 version of postgre, and the aggregate function is
this:
CREATE AGGREGATE "grafika"."agg_pov_sphere" (numeric, numeric, numeric,
numeric, varchar) (
SFUNC = "grafika"."_tran_pov_sphere",
STYPE = "varchar",
INITCOND = ''
);

when i change the header to this CREATE AGGREGATE "grafika"."agg_pov_sphere"
(x numeric, y numeric, z numeric, rad numeric, pigment varchar) (
i get syntax error.

Thanx in advance
--
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Viktor Bojović
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Wherever I go, Murphy goes with me