Keep alive together - call for use cases

[Mark Knoop]

I've been working on some patches to enable more flexible uses for the
Keep_alive_together_engraver. Whilst I've achieved my particular use
case, David K is (rightly) keen that any changes might also address (or
at least not rule out) other potential improvements.

(If you're interested in the code, look here:
https://codereview.appspot.com/308910043/)

So if I could ask users for any situations where they can imagine this
engraver helping, that currently don't work, or don't work easily. I'll
start by describing my case, which came up on the list recently.

Duplicating rehearsal, tempo, volta marks etc at multiple vertical
positions in an orchestral score is possible with a custom MarkLine
context (http://lsr.di.unimi.it/LSR/Item?id=1010), however this
technique currently doesn't work in a Frenched score, as there may be
passages where e.g. the brass are tacet so two MarkLines would be shown
together. My fix makes it possible to set a context (Staff or MarkLine)
to be kept alive only while there is another Staff in the group.

See also the temporary divisi staves example here:
http://lilypond.org/doc/v2.19/input/regression/a6/lily-436997bb.ly
as another use of the engraver.

Many thanks for your input.

--
Mark Knoop