public function multipleOptions($key = null)
    {
        if ($this->multipleOptions === null) {
            $this->multipleOptions = $this->defaultMultipleOptions();
        }

        if (is_string($key)) {
            if (isset($this->multipleOptions[$key])) {
                return $this->multipleOptions[$key];
            } else {
                return null;
            }
        }

        return $this->multipleOptions;
    }