code: private function createMakeDefaultForm(CustomFieldsGroup $group = null)
      {
          return $this->createFormBuilder($group, array(
                      'method' => 'POST',
                      'action' => $this->generateUrl('customfieldsgroup_makedefault')
                  ))
                  ->add('id', 'hidden')
                  ->add('submit', 'submit', array('label' => 'Make default'))
                  ->getForm();
      }
docs: create a form to make the group default
  
  @param CustomFieldsGroup $group
  @return \Symfony\Component\Form\Form