NoteListParser
receives a cons cell of Scheme language which contains
a list of association lists ( called note list ), and converts it to
MIDI data.NoteListParserElement
is a representation of a type of
notes on the note list.Metro.registerTrack(MetroTrack)
and
Metro.unregisterTrack(MetroTrack)
has responsibility to call
Metro.notifyTrackChange()
method after calling.