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

        return $this;
    }