the matrix constraint doesn't depend on the variable   and also to carefully distinquish zero matrix (<tt>sparse(m,m)</tt>) and empty   matrix <tt>[]</tt>.
               </li>
               <li>a 2nd derivative of the <tt>k</tt>-th nonlinear matrix constraint (<tt>k=1..NANLN</tt>)   w.r.t. <tt>i,j</tt>-th variables (<tt>i,j=1..Nx+NYnnz</tt>) as a dense or sparse   symmetric matrix of the appropriate dimension or empty matrix <tt>[]</tt>.   <tt>mconhess()</tt> will be called only with the indices <tt>i,j</