aggregate version of first_value function?

We have window function version of first_value(),
but aggregate version looks useful to write queries something like:

=# CREATE TABLE obj (id integer, pos point);
=# SELECT X.id,         first_value(Y.id ORDER BY X.pos <-> Y.pos) AS neighbor  FROM obj X, obj Y  GROUP BY X.id;

Is it reasonable? Or, do we have alternative ways for the same purpose?

-- 
Itagaki Takahiro