Adding indicators to staves which get split after a break

This snippet defines the commands \splitStaffBarLine, \convUpStaffBarLine, and \convDownStaffBarLine. These add arrows at a bar line to denote that several voices sharing a staff will each continue on a staff of their own in the next system, or that voices split in this way recombine.

Note that the implementation in this snippet draws dimensionless arrows into the right margin. For normal printing, this doesn't cause problems. However, it is necessary to increase the bounding box horizontally if you render the code as an image to avoid cropping, as demonstrated below.

\version "2.24"

#(define-markup-command (arrow-at-angle layout props angle-deg length fill)
   (number? number? boolean?)
   (let* (
           (PI-OVER-180 (/ (atan 1 1) 34))
           (degrees->radians (lambda (degrees) (* degrees PI-OVER-180)))
           (angle-rad (degrees->radians angle-deg))
           (target-x (* length (cos angle-rad)))
           (target-y (* length (sin angle-rad))))
     (interpret-markup layout props
       (markup
        #:translate (cons (/ target-x 2) (/ target-y 2))
        #:rotate angle-deg
        #:translate (cons (/ length -2) 0)
        #:concat (#:draw-line (cons length 0)
                   #:arrow-head X RIGHT fill)))))

splitStaffBarLineMarkup = \markup \with-dimensions #'(0 . 0) #'(0 . 0) {
  \combine
  \arrow-at-angle #45 #(sqrt 8) ##t
  \arrow-at-angle #-45 #(sqrt 8) ##t
}

splitStaffBarLine = {
  \once \override Staff.BarLine.stencil =
  #(lambda (grob)
     (ly:stencil-combine-at-edge
      (ly:bar-line::print grob)
      X RIGHT
      (grob-interpret-markup grob splitStaffBarLineMarkup)
      0))
  \break
}

convDownStaffBarLine = {
  \once \override Staff.BarLine.stencil =
  #(lambda (grob)
     (ly:stencil-combine-at-edge
      (ly:bar-line::print grob)
      X RIGHT
      (grob-interpret-markup grob #{
        \markup\with-dimensions #'(0 . 0) #'(0 . 0) {
          \translate #'(0 . -.13)\arrow-at-angle #-45 #(sqrt 8) ##t
        }#})
      0))
  \break
}

convUpStaffBarLine = {
  \once \override Staff.BarLine.stencil =
  #(lambda (grob)
     (ly:stencil-combine-at-edge
      (ly:bar-line::print grob)
      X RIGHT
      (grob-interpret-markup grob #{
        \markup\with-dimensions #'(0 . 0) #'(0 . 0) {
          \translate #'(0 . .14)\arrow-at-angle #45 #(sqrt 8) ##t
        }#})
      0))
  \break
}

\paper {
  indent = 10\mm
  short-indent = 10\mm
  line-width = 8\cm
}

separateSopranos = {
  \set Staff.instrumentName = "AI AII"
  \set Staff.shortInstrumentName = "AI AII"
  \splitStaffBarLine
  \change Staff = "up"
}
convSopranos = {
  \convDownStaffBarLine
  \change Staff = "shared"
  \set Staff.instrumentName = "S A"
  \set Staff.shortInstrumentName = "S A"
}

sI = {
  \voiceOne
  \repeat unfold 4 f''2
  \separateSopranos
  \repeat unfold 4 g''2
  \convSopranos
  \repeat unfold 4 c''2
}
sII = {
  s1*2
  \voiceTwo
  \change Staff = "up"
  \repeat unfold 4 d''2
}
aI = {
  \voiceTwo
  \repeat unfold 4 a'2
  \voiceOne
  \repeat unfold 4 b'2
  \convUpStaffBarLine
  \voiceTwo
  \repeat unfold 4 g'2
}
aII = {
  s1*2
  \voiceTwo
  \repeat unfold 4 g'2
}
ten = {
  \voiceOne
  \repeat unfold 4 c'2
  \repeat unfold 4 d'2
  \repeat unfold 4 c'2
}
bas = {
  \voiceTwo
  \repeat unfold 4 f2
  \repeat unfold 4 g2
  \repeat unfold 4 c2
}

\markup \pad-x #3  % avoid cropping
  \score {
    <<
      \new ChoirStaff <<
        \new Staff = up \with {
          instrumentName = "SI SII"
          shortInstrumentName = "SI SII"
        } {
          s1*4
        }

        \new Staff = shared \with {
          instrumentName = "S A"
          shortInstrumentName = "S A"
        } <<
          \new Voice = sopI \sI
          \new Voice = sopII \sII
          \new Voice = altI \aI
          \new Voice = altII \aII
        >>
        \new Lyrics \with {
          alignBelowContext = up
        }
        \lyricsto sopII { e f g h }
        \new Lyrics \lyricsto altI { a b c d e f g h i j k l }

        \new Staff = men \with {
          instrumentName = "T B"
          shortInstrumentName = "T B"
        } <<
          \clef F
          \new Voice = ten \ten
          \new Voice = bas \bas
        >>
        \new Lyrics \lyricsto bas { a b c d e f g h i j k l }
      >>
    >>

    \layout {
      \context {
        \Staff \RemoveEmptyStaves
        \override VerticalAxisGroup.remove-first = ##t
      }
    }
}