public function offsetGet($offset)
    {
        if (! $this->has($offset)) {
            throw new UnknownIdentifierException($offset);
        }

        if (!isset($this->_frozen[$offset])) {
            $entry = &$this->_store[$offset];
            if (is_callable($entry)) {
                $entry = $entry($this);
            }
            $this->_frozen[$offset] = &$entry;
        }

        return $this->_frozen[$offset];
    }