above, the
   function <code class="function">float8mi</code> that underlies the regular <code class="type">float8</code>
   minus operator can be used; but for any other subtype, some type
   conversion would be necessary.  Some creative thought about how to
   represent differences as numbers might be needed, too.  To the greatest
   extent possible, the <code class="literal">subtype_diff</code> function should agree with
   the sort ordering implied by the selected operator class and collation;
   that i