public function setId($id = '')
    {
        if (is_string($id) && (empty($id) === true || Format::isIdentifier($id, false) === true)) {
            $this->id = $id;
        } else {
            $msg = "The 'id' argument of a body element must be a valid identifier or an empty string";
            throw new InvalidArgumentException($msg);
        }
    }