public function multiply($number = null)
    {
        if ($number === null) {
            $this->internalSetOperationModeWithHistory('multiply');
        } elseif ($number instanceof Closure) {
            $this->internalSetOperationModeWithHistory('multiply');
            return $this->group($number);
        } else {
            $this->performOperationOnExpressionEngine('multiply', $number);
        }

        return $this;
    }