add method
Add element to this voice.
The element is stored as-is. Note.voice and Chord.voice are final,
so this method cannot back-fill the voice number: it has to be supplied
at construction time, e.g. Note(pitch: ..., duration: ..., voice: 2).
validate reports elements whose voice number disagrees with number.
Implementation
void add(MusicalElement element) {
elements.add(element);
}