Re:Keep alive together - call for use cases

[Mats Bengtsson]

> 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.
I recently typeset a concerto grosso from the baroque period, where the 
solo and tutti parts are identical during long sections and then they 
differ for some bars, every here and there. When entering the parts, I 
used tags to handle the differences so producing the separate parts 
works without problem. However, in the conductors score, I would prefer 
to only use a single stave per instrument when the solo and tutti play 
in unison, but individual staves when they differ. I managed to handle 
this using remove-layer as long as there's only a single instrument 
group that contains and occasional solo part, but if for example both 
violin 1 and violin 2 have a solo plus a tutti, then I didn't manage to 
get it to work correctly. The following example shows my current 
attempt. As can be seen, the split between solo and tutti parts happens 
both for violin 1 and 2 as soon as one of them is split. For example, in 
the second score line, I would like to see Vi I solo, Vi I tutti but 
only one stave with Vi II.

\version "2.19.40"

viI = \relative c'' {
  \repeat unfold 8 {c4 d e d | }
  \tag #'viIsolo { \repeat unfold 4 {c16 d e f g f e d }}
  \tag #'viItutti { \repeat unfold 2 {c1 }}
  \repeat unfold 24 {c4 d e d | }
}

viII = \relative c' {
  \repeat unfold 16 {g4 c a b | }
  \tag #'viIIsolo { \repeat unfold 4 {g16 c e c g c e c }}
  \tag #'viIItutti { \repeat unfold 2 {g1 }}
  \repeat unfold 16 {g4 c a b | }
}

\paper{
  left-margin = 25\mm
}

\score{
  \new StaffGroup <<
    \new GrandStaff <<
      \new Staff \with {instrumentName = #"Violin I solo"
            shortInstrumentName = "Vi I solo"
            \override VerticalAxisGroup.remove-empty = ##t
            \override VerticalAxisGroup.remove-first = ##t
            \override VerticalAxisGroup.remove-layer = 1 } \pushToTag 
#'viIsolo \unset Staff.keepAliveInterfaces \appendToTag #'viIsolo \set 
Staff.keepAliveInterfaces = #'() \keepWithTag #'viIsolo {\set 
Staff.keepAliveInterfaces = #'() \viI }
      \new Staff \with {instrumentName = #"Violin II solo"
            shortInstrumentName = "Vi II solo"
            \override VerticalAxisGroup.remove-empty = ##t
            \override VerticalAxisGroup.remove-first = ##t
            \override VerticalAxisGroup.remove-layer = 1 } \pushToTag 
#'viIIsolo \unset Staff.keepAliveInterfaces \appendToTag #'viIIsolo \set 
Staff.keepAliveInterfaces = #'() \keepWithTag #'viIIsolo {\set 
Staff.keepAliveInterfaces = #'() \viII }
    >>
    \new GrandStaff <<
      \new Staff \with {instrumentName = #"Violin I tutti"
            shortInstrumentName = "Vi I tutti"
            \override VerticalAxisGroup.remove-empty = ##t
            \override VerticalAxisGroup.remove-first = ##t
            \override VerticalAxisGroup.remove-layer = 1  } \pushToTag 
#'viItutti \unset Staff.keepAliveInterfaces \appendToTag #'viItutti \set 
Staff.keepAliveInterfaces = #'() \keepWithTag #'viItutti {\set 
Staff.keepAliveInterfaces = #'() \viI }
      \new Staff \with {instrumentName = #"Violin I"
            shortInstrumentName = "Vi I"
            \override VerticalAxisGroup.remove-layer = 2  } \pushToTag 
#'viIsolo \unset Staff.keepAliveInterfaces \appendToTag #'viIsolo \set 
Staff.keepAliveInterfaces = #'() \keepWithTag #'viIsolo {\set 
Staff.keepAliveInterfaces = #'() \viI }
      \new Staff \with {instrumentName = #"Violin II tutti"
            shortInstrumentName = "Vi II tutti"
            \override VerticalAxisGroup.remove-empty = ##t
            \override VerticalAxisGroup.remove-first = ##t
            \override VerticalAxisGroup.remove-layer = 1  } \pushToTag 
#'viIItutti \unset Staff.keepAliveInterfaces \appendToTag #'viIItutti 
\set Staff.keepAliveInterfaces = #'() \keepWithTag #'viIItutti {\set 
Staff.keepAliveInterfaces = #'() \viII }
      \new Staff \with {instrumentName = #"Violin II"
            shortInstrumentName = "Vi II"
            \override VerticalAxisGroup.remove-layer = 2  } \pushToTag 
#'viIIsolo \unset Staff.keepAliveInterfaces \appendToTag #'viIIsolo \set 
Staff.keepAliveInterfaces = #'() \keepWithTag #'viIIsolo {\set 
Staff.keepAliveInterfaces = #'() \viII }
    >>
  >>
  \layout{
    \context{
      \Score
      \consists "Keep_alive_together_engraver"
    }
  }
}


I have to admit that I don't fully understand the 
Keep_alive_together_engraver so it may certainly be that use case 
already can be handled. Also, the input could obviously look less messy 
by introducing some macros, but it's my first quic attempt.

    /Mats