public function offsetExists($identifier)
    {
        $keys   = $this->_parseIdentifier($identifier);
        $last   = count($keys)-1;
        $data   = &$this->_data;
        $result = false;

        foreach($keys as $index => $key)
        {
            if (!array_key_exists($key, $data)) {
                break;
            }

            if ($index < $last) {
                $data =& $data[$key];
            }
            else $result = true;
        }

        return $result;
    }