aggregate function

I am trying to make aggregate function of existing function which looks like
this.
CREATE OR REPLACE FUNCTION "grafika"."pov_sphere" (x numeric, y numeric, z
numeric, rad numeric, pigment varchar) RETURNS varchar AS
$body$
DECLARE
_pov varchar;
BEGIN
_pov:='sphere {<'||x||','||y||','||z||'>,'||rad||' '||pigment ||'}';
return _pov;
END;
$body$
LANGUAGE 'plpgsql'
IMMUTABLE
RETURNS NULL ON NULL INPUT
SECURITY INVOKER
COST 100;

Aggregate should concatenate results of pov_sphere using this function
below.
CREATE OR REPLACE FUNCTION "public"."concat" (varchar, varchar) RETURNS
varchar AS
$body$
DECLARE
t varchar;
BEGIN
IF character_length($1) > 0 THEN
t = $1 || $2;
ELSE
t = $2;
END IF;
RETURN t;
END;
$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;

I tried to write this part below, but something is wrong (ERROR: function
grafika.pov_sphere(character varying, numeric, numeric, numeric, numeric,
character varying) does not exist) so I wanted to ask if someone knows how
to solve this problem.

CREATE AGGREGATE "grafika"."agg_pov_sphere" (NUMERIC, NUMERIC, NUMERIC,
NUMERIC, VARCHAR) (
SFUNC = "grafika"."pov_sphere",
STYPE = "varchar",
FINALFUNC = "public"."grp_concat"
);

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