Miners can 'dig in' on z-levels below/above them

I would expect that they can't complete the 'dig in' designations on floors above or below them. Or maybe unless there is an open tile above them (ie, it has been dug out completely with 'dig down') and the 'dig in' is immediately next to that (though that's still a fairly difficult manoeuvre!).

Though it would be great if we could choose to 'dig out' the entire tile above them the same way we can dig the one below - that they're standing on).