unexpected \unfoldRepeats behavior

[Mark Polesky]

I tried to answer a question on -user but hit a brick wall.
\unfoldRepeats failed to work when the music block was funneled from 2
separate expressions. Can someone explain this to me? Why does the
following code work for voiceA but not for voiceB?

Thanks.
- Mark

_____________________________________


\version "2.13.1"

voiceA = \new Voice = "A" \relative {
  \time 1/4
  \repeat volta 2 { a' }
  \alternative { { b } { c } }
}


% voiceB is built from 2 separate expressions funneled into one voice,
% but otherwise ought to be identical to voiceA, it seems:
voiceBrepeats = {
  \time 1/4
  \repeat volta 2 { s }
  \alternative { { s } { s } }
}
voiceBnotes = \relative { \time 1/4 a' b c }
voiceB = \new Voice = "B" { << \voiceBrepeats \voiceBnotes >> }


% using \unfoldRepeats produces the expected MIDI output with voiceA,
% but not with voiceB. Why?
\score {
  \unfoldRepeats \voiceA % change to \voiceB to test.
  \midi { }
}