SCREEN, SCREENVISIBLE and SCREENINDEX Functions missing something

So we have this documentation:

However, how the *** do you get SCREENVISIBLE(FRONT - 1, ... ) ?

We have FRONT for the screen containing (the most of) the front window.
and BACK for the first from the left not containing (the most of) the front window.

But why do we not have an index capability based on the screen containing the front window minus or plus 1,2,3 ?

SCREENINDEX returns the index of a screen. Have you tried SCREENVISIBLE(SCREENINDEX(Front) - 1,...)?

I don't have a second monitor here, so I'm afraid I can't test this and be certain. You might also have to try/catch for when your left-most display has the frontmost window...