Multidimensional session data

[eluser]spheroid[/eluser]
How is it possible to do the following refactored for CI? Having difficulty compared to the documentation:

1. Increase the value by 1

Code:

$_SESSION['cart_product_qty'][$i]++;

2. Compare a multidimensional session array key

Code:

if ($_SESSION['cart_product_id'][$i] == $product_id) ...

3. Set a session variable using an array key

Code:

$_SESSION['cart_product_qty'][] = 1;

4. Display a multidimensional session variable

Code:

echo $_SESSION['cart_product_qty'][$i];

I'm not sure if it's possible to use these type of multidimensional session variables??