code: Rule FileField() {
  		return FirstOf(FieldArea(), FieldBook(), FieldComposer(),
  				FieldDiscography(), FieldFile(), FieldGroup(),
  				FieldHistory(), FieldNotes(), FieldOrigin(),
  				FieldRhythm(), FieldSource(), FieldUserdefPrint(),
  				FieldUserdefPlay(), FieldTranscription(),
  				UnusedField()
  				//FieldLength(), FieldMeter(), FieldParts(),
  				//FieldTempo(), FieldWords(), FieldKey()
  			).label(FileField);
  	}
docs: Default values for the whole file - all fields except number, title and
  voice
  
  file-field ::= field-area / field-book / field-composer /
  field-discography / field-file / field-group / field-history /
  field-length / field-meter / field-notes / field-origin / field-parts /
  field-tempo / field-rhythm / field-source / field-userdef-print /
  field-userdef-play / field-words / field-transcription / field-key /
  unused-field