public static function addChild(&$target, $primary_key, $level, $max_depth, &$children = array())
    {
        if (isset($children[$target[$primary_key]])) {
            $target['children'] = $children[$target[$primary_key]];
            if ($level + 1 < $max_depth) {
                foreach ($target['children'] as &$child) {
                    SC_Utils_Ex::addChild($child, $primary_key, $level++, $max_depth, $children);
                }
            }
        }
    }