code: Rule Xcom() {
  		return FirstOf(XcomStaff(), XcomMeasurenb(), XcomLayout(),
  				XcomMargins(), XcomMidi(), XcomOther(), XcomText())
  			.label(Xcom);
  	}
docs: xcom ::= xcom-staff / xcom-measurenb / xcom-text / xcom-layout
  / xcom-margins / xcom-midi / xcom-other
  <p>
  more need to be defined