code: Rule TuneField() {
  		return FirstOf(
  			FieldArea(), FieldBook(), FieldComposer(),
  			FieldDiscography(), FieldGroup(), FieldHistory(), FieldLength(),
  			FieldMeter(), FieldNotes(), FieldOrigin(), FieldPart(),
  			FieldTempo(), FieldRhythm(), FieldSource(), FieldTitle(),
  			FieldKey(), FieldVoice(), FieldWords(), FieldLyrics(),
  			FieldInstruction(), UnusedField()
  		).label(TuneField);
  	}
docs: tune-field ::= field-area / field-book / field-composer
  / field-discography / field-group / field-history / field-length
  / field-meter / field-notes / field-origin / field-part
  / field-tempo / field-rhythm / field-source / field-title
  / field-voice / field-words / field-lyrics / field-transcription
  / field-key / unused-field