\version "2.19.82"

#(define-markup-command (arrow-at-angle layout props angle length fill)
  (number? number? boolean?)
  (ly:stencil-rotate
    (interpret-markup layout props
      (markup #:concat (#:draw-line (cons length 0)
                        #:hspace -0.1
                        #:arrow-head X RIGHT fill)))
    angle -1 0))

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

splitStaff = #(define-music-function (grob-name) (string?)
  #{ { \once \override Staff.$grob-name stencil =
          #(grob-transformer 'stencil (lambda (grob orig)
            (if (= LEFT (ly:item-break-dir grob))
              (ly:stencil-combine-at-edge
                orig X RIGHT
                (grob-interpret-markup grob splitStaffMarkup)
                (if (equal? "TimeSignature" grob-name) 0.5 0))
              orig)))
        \break } #} )

convDownStaff = #(define-music-function (grob-name) (string?)
  #{ { \once \override Staff.$grob-name stencil =
          #(grob-transformer 'stencil (lambda (grob orig)
            (if (= LEFT (ly:item-break-dir grob))
              (ly:stencil-combine-at-edge
                orig X RIGHT
                (grob-interpret-markup grob #{
                  \markup
                    \with-dimensions #'(0 . 0) #'(0 . 0)
                    \arrow-at-angle #-45 #(sqrt 8) ##t #})
                (if (equal? "TimeSignature" grob-name) 0.5 0))
              orig)))
        \break } #} )

convUpStaff = #(define-music-function (grob-name) (string?)
  #{ { \once \override Staff.$grob-name stencil =
          #(grob-transformer 'stencil (lambda (grob orig)
            (if (= LEFT (ly:item-break-dir grob))
              (ly:stencil-combine-at-edge
                orig X RIGHT
                (grob-interpret-markup grob #{
                  \markup
                    \with-dimensions #'(0 . 0) #'(0 . 0)
                    \arrow-at-angle #45 #(sqrt 8) ##t #})
                (if (equal? "TimeSignature" grob-name) 0.5 0))
              orig)))
        \break } #} )

\paper {
  ragged-right = ##t
  short-indent = 10\mm
}

separateSopranos = {
  \set Staff.instrumentName = "AI AII"
  \set Staff.shortInstrumentName = "AI AII"
  \tag #'no-time-change \splitStaff BarLine
  \tag #'time-change \splitStaff TimeSignature
  \change Staff = "up"
}
convSopranos = {
  \tag #'no-time-change \convDownStaff BarLine
  \tag #'time-change \convDownStaff TimeSignature
  \change Staff = "shared"
  \set Staff.instrumentName = "S A"
  \set Staff.shortInstrumentName = "S A"
}

sI = {
  \voiceOne
  \repeat unfold 4 f''2
  \separateSopranos
  \tag #'time-change \time 4/2
  \repeat unfold 4 g''2
  \convSopranos
  \tag #'time-change \time 4/4
  \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
  \tag #'no-time-change \convUpStaff BarLine
  \tag #'time-change \convUpStaff TimeSignature
  \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
}

\score { \keepWithTag #'(no-time-change)
  <<
    \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
    }
  }
}

\score { \keepWithTag #'(time-change)
  <<
    \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
    }
  }
}