I haven't looked at the code, but I suspect they're just using display:none to hide elements. There's no reason you couldn't use an array as a stack so that you

I haven't looked at the code, but I suspect they're just using display:none to hide elements. There's no reason you couldn't use an array as a stack so that you could keep track of the identifiers for the elements that have been hidden and then allow an undo mechanism. For elements without an id you'd add something of your own, probably a cssclass. An undo system is not too difficult to implement.