l be counted with indices starting with <tt>(Nx+1)</tt> (<tt>Nx</tt> = number of scalar variables), using the ordering <tt>Y{1}, Y{2}, ..., Y{NY}</tt>. Every <tt>Y{i}</tt> is assumed to be symmetric and thus only the <b>lower triangle is vectorized column-wise</b>. If any callback functions described below asks for derivatives with respect to <tt>k</tt> (<tt>k&gt;Nx</tt>), it means it's one of the variables in one of the matrices <tt>Y{i}</tt>.
         </p>
         <p>Call back functions are used to evalu