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