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