public function incrementQuantityAt($cartItemIndex, $quantity = 1)
    {
        $this->existenceCheckFor($cartItemIndex);

        $this->items[$cartItemIndex]->quantity += $quantity;

        $this->cartDriver->setCartItemQuantity(
            $this->items[$cartItemIndex]->id,
            $this->items[$cartItemIndex]->quantity
        );

        return $this->cartUpdates();
    }