public function bt($s)
    {
        if (is_array($s)) {
            $out = array();
            foreach ($s as $ss) {
                $out[] = $this->bt($ss);
            }
            return $out;
        }

        if (!$this->bt
            || is_object($s)
            || $s === '*'
            || strpos($s, '.') !== false
            || strpos($s, '(') !== false
            || strpos($s, $this->bt) !== false
        ) {
            return $s;
        }

        return $this->bt.$s.$this->bt;
    }