musicUp = \relative c'' {
  \time 4/4
  a4 c4.( g8) a4 |
  g4 e' g,( a8 b) |
  c b a2.
}

musicDown = \relative c'' {
  g4 e4.( d8) c4 |
  r2 g'4( f8 e) |
  d2 \stemDown a
}

\score {
  <<
    \new Staff \with {
      instrumentName = "standard polyphony"
    } << \musicUp \\ \musicDown >>

    \new Staff \with {
      instrumentName =
        \markup { \typewriter "\\partCombine" without text}
      printPartCombineTexts = ##f
    } \partCombine \musicUp \musicDown

    \new Staff \with {
      instrumentName =
        \markup { \typewriter "\\partCombine" with text}
    } \partCombine \musicUp \musicDown
  >>

  \layout {
    indent = 6.0\cm
    \context {
      \Score
      % Setting this to a large value avoids a bar line at the
      % beginning that would connect the three staves otherwise.
      \override SystemStartBar.collapse-height = 30
    }
  }
}