For instance: 0 = empty (walkable tile); X = support beamed tile; R = rock tile.
In this example you can collapse all of the 0 tiles. (You can not collapse the tiles already fill with rock.)
<style>h6 {page-break-before: always;}RX0XRRR0RRRR0XRRR0XR</style>
You could collapse all 0 (empty) tiles making it look like this:
RRRRRRXRXRRRRRRRRRXR
After which you could mine out all of the R tiles again. But not the X, unless you are mayor of that tile.