public function setBase($base)
    {
        if (is_int($base) === true || Format::isVariableRef($base) === true) {
            $this->base = $base;
        } else {
            $msg = "The 'base' argument must be an integer or a variable reference, '" . $base . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }