public function setN($n)
    {
        if (is_int($n) || (gettype($n) === 'string' && Format::isVariableRef($n))) {
            $this->n = $n;
        } else {
            $msg = "The n attribute must be an integer or a variable reference, '" . gettype($n) . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }