ss="literal">subtype_diff</code>,
   function.  (The index will still work without <code class="literal">subtype_diff</code>,
   but it is likely to be considerably less efficient than if a difference
   function is provided.)  The subtype difference function takes two input
   values of the subtype, and returns their difference
   (i.e., <em class="replaceable"><code>X</code></em> minus <em class="replaceable"><code>Y</code></em>) represented as
   a <code class="type">float8</code> value.  In our example 