DrumVoice Polyphony and MIDI

[Stefano Antonelli]

Greetings List,

I'm trying to understand why one syntax results in MIDI output, while
another syntax does not.  I'm new to lilypond and clearly missing
something.

Below is the working case:

tagCymbals = \drummode { \voiceOne   cymc4 cymc4 cymc4 cymc4 }
tagDrums =   \drummode { \voiceTwo   s4    sn4   s4    sn4   }
tagKick =    \drummode { \voiceThree bd4   s4    bd4   s4    }

\score {
  \new DrumStaff <<
    \set Staff.instrumentName = #"Drums"
    \new DrumVoice { \tagCymbals }
    \new DrumVoice { \tagDrums }
    \new DrumVoice { \tagKick }
  >>
  \layout { }
  \midi {
    \tempo 4 = 84
  }
}
\version "2.18.2"

That is not always a convenient format for working with drum parts
though.  I would like to be able to use a syntax more like the
following:

test = \drummode {
  <<
  { \voiceOne cymc4 cymc4 cymc4 cymc4 }
  { \voiceOne s4    sn4   s4    sn4   }
  \\
  { \voiceTwo bd4   s4    bd4   s4    }
  >>
}

\score {
  \new DrumStaff {
    <<
      \set Staff.instrumentName = #"Drums"
      \new DrumVoice {
        \test
%       \morePolyphonicPieces
      }
    >>
  }
  \layout { }
  \midi {
    \tempo 4 = 84
  }
}
\version "2.18.2"

The above code generates a score just fine, but the MIDI file is empty
(aside from a header).

I don't actually know much about MIDI.  I'm just using the play button
in Frescobaldi.  It's helpful when transcribing.

Thanks,
Stef