"shuffle" A Deck Of Papers - Elements On A Page ?

OKay, I preface this with : "I am not new to code, but have only limited experience with JS."

That said, I want to use JS to shuffle elements on a page. Not randomize them (as iPods have redefined the word shuffle.).

For example, I have divs A B C D E. When you visit the page they are in this order:

When you click the button for "E" I want it to come to the front. Causing the display to be:

If you then clicked the button for "B" it would end up:

I hope thats clear enough, its a difficult concept to articulate. So, depending on the order in which the user clicks the buttons, the stack is shuffled.

Now, I've successfully been able to:

1) set them in the page using CSS in a stack with zIndex. So, when first visiting the page they have a zIndex of A=z5 B=z4 C=z3 etc.

2) I've used the following code on a button to bring to front when clicked.

This will bring any of the divs to the front when clicked. But say I click B, then C. Since this function is assigning a specific zIndex they wont overlap anymore.

I think what I need is some sort of add 5 to zIndex function. (Has to be 5 in the event that the item you click is on the bottom of the stack of 5)

So far I've tried:

I imagine that first one is way off base, but I really thought the second one would work. Found it while searching here. If someone could point me in the right direction, or better yet EXPLICITLY explain what to do, I'd be forever indebted.