creating a new aggregate function

Hi,

I'm trying to implement an aggregate function to calculate the average
angle from one or more angles and corresponding magnitudes.  So my first
step is to design a function that decomposes the angles and magnitudes
and returns the corresponding x and y vectors, and the following works
does this:

---<--------------------cut here---------------start------------------->---
CREATE OR REPLACE FUNCTION decompose_angle(IN angle numeric, IN magnitude numeric, OUT x numeric, OUT y numeric)
RETURNSrecord AS
 
$BODY$
BEGINx := sin(radians(angle)) * magnitude;y := cos(radians(angle)) * magnitude;
END;
$BODY$ LANGUAGE plpgsql STABLE COST 100;
ALTER FUNCTION decompose_angle(numeric, numeric) OWNER TO sluque;
COMMENT ON FUNCTION decompose_angle(numeric, numeric) IS 'Decompose an angle and magnitude into x and y vectors.';
---<--------------------cut here---------------end--------------------->---

Before moving on to writing the full aggregate, I'd appreciate any
suggestions to understand how to go about writing an aggregate for the
above, that would return the average x and y vectors.

Cheers,

-- 
Seb