"code">nan</code>
   as different from any other float value, including itself;
   while <code class="code">compare</code> treats <code class="code">nan</code> as equal to itself and less than any
   other float value.  This treatment of <code class="code">nan</code> ensures that <code class="code">compare</code>
   defines a total ordering relation.
<p>

   <code class="code">compare</code> applied to functional values may raise <code class="code"><span class="constructor">Invalid_argument</span></code>.
 