I understand (more and more these days) the need for some way of being reminded of the words (and sometimes the tune, or at least the matching 'B' part) and I am amazed that there appears to be an "app" for an index of first lines, but why the .... does a musician need a 'black box' to tell that s/he's not in tune with the next person? If you can't tell if you're not in tune it must be time to give up (unless you're into modern classical that is!) Next thing we'll each be watching our own metronome. Now that could produce an interesting effect.