[development] form_set_value()

Hi all, I have a problem with form_set_value(). I thought that this would set a value in the form, but the attached example module doesn't work as I expect. It is a simple form with radios and in the _validate() I want to select a different radio button if t('Next') is clicked. Can someone help me? I've tried on #drupal, but noone was helpful (although chx tried to be). Thanks, Robrecht