midi doesn't work with custom Voice contexts

[Janek Warchoł]

Hi,

apparently somewhere in midi generation there's a hard-coded "Voice"
context, and midi doesn't want to work with custom-defined contexts.
This appears to be similar to
http://lists.gnu.org/archive/html/lilypond-user/2013-09/msg00261.html

Example code is at
http://github.com/openlilylib/snippets/blob/master/templates/predefined-instruments/midi-bug.ly

best,
Janek