$form['comment_control_settings'] = array(
    '#type' => 'fieldset',
    '#title' => t('Comment Control Form'),
    '#collapsible' => TRUE,
    '#tree' => TRUE,
  );
  $options = array('mode' => 'Mode (flat or threaded)','order' => 'Default Order (newest or oldest first)','limit' => 'Comments per Page');
  $form['comment_control_settings']['options'] = array(
    '#title' => 'What parts of the comment controls should be displayed to the user',
    '#collapsible' => true,
    '#type' => 'checkboxes',
    '#default_value' => array('mode','order','limit'),
    '#options' => $options,
  );