[xmonad] user-visible windows

Hi all, I'm writing an extension and would like to know if there's some way to get a set of all windows visible to the user? I'm aware of StackSet.visible and StackSet.current. The windows obtained from these are not necessarily visible to the user- some may be totally obscured by others. I could get a list of the windows in z-order and calculate this myself, but was wondering whether anything existed that might suit my needs? Thanks in advance, Matt