Bug fix: a VoiceDef may now contain a single rest.
Bug fix: a VoiceDef may now contain a single rest.

A piece is defined as a list of voiceDefs, each of which
starts at MsPosition=0 and has the same MsDuration.
There is at least one MidiChordDef at the start of the
piece, and at least one MidiChordDef ends at its end.

notator/Moritz