s, its result should be positive whenever its first argument is
   greater than its second according to the sort ordering.
  </p><p>
   A less-oversimplified example of a <code class="literal">subtype_diff</code> function is:
  </p><pre class="programlisting">
CREATE FUNCTION time_subtype_diff(x time, y time) RETURNS float8 AS
'SELECT EXTRACT(EPOCH FROM (x - y))' LANGUAGE sql STRICT IMMUTABLE;

CREATE TYPE timerange AS RANGE (
    subtype = time,
    subtype_diff = time_subtype_diff
);

SELECT '[11:10, 23:0