the problem</p><pre> problem = penlab(problem_data);</pre><p>Then you can optionally edit some stuff before calling the solver itself, for example, set up the user data to be passed to
            the callbacks, set starting point, set optional parameters,... Typically you just change the appropriate bits in the structure:
         </p><pre> problem.xinit = ...
 problem.Yinit = ...
 problem.opts.chosen_option = ...</pre><p>or</p><pre> new_opts = structure_something
 problem.opts = new_opts;</pre><p>Typicall