/**
  * Return list of sub-nodes for 'node'.
  * @param FOLDER $node
  * @return FOLDER[]
  * @access private
  */
 public function sub_nodes($node)
 {
     return $node->sub_folders();
 }
 /**
  * @return FOLDER[]
  * @param FOLDER $obj
  * @access private
  */
 protected function _obj_sub_objects($obj)
 {
     return $obj->sub_folders();
 }