    <!-- return types: from API spec., p. 89, 6.13.6 Relational Functions; 
         and OpenCL C spec., p. 20, 9.4.5 Half - Relational Functions.
         These words apply to: isequal, isgreater, isgreaterequal, isless, 
         islessequal, and islessgreater. -->

    <para>
      The function returns 0 if either argument is not a number (NaN).
    </para>

<!-- 13-Dec-2013, rev. 19 -->