public function get($identifier = null)
    {
        $result = null;

        if (null === $identifier) {
            $result = self::$lookup;
        } elseif ('doozr.registry' === $identifier) {
            $result = $this;
        } else {
            if (true === isset(self::$lookup[$identifier])) {
                $result = self::$references[self::$lookup[$identifier]];
            }
        }

        return $result;
    }