y sparse) vector w.r.t. all variables (even matrix ones), if objfun doesn't depend on <tt>Y</tt> it can be just <tt>Nx</tt> long, otherwise <tt>Nx+NYnnz</tt></p>
         <p>Callbacks can be defined without <tt>Y</tt>, userdata as the parameters (really?) but they always need to return userdata. If userdata structure is not used, it should
            return <tt>[]</tt>.
         </p>
         <p>The following 3 callbacks are used to evaluate the objective function:</p><pre>   function [f,   userdata] = obj