code: Rule TElem() {
  		return FirstOf(
  			WSP(), ChordOrText(), Gracing(), GraceNotes(), BrokenRhythm(),
  			SlurBegin(), SlurEnd()
  		).label(TElem);
  	}
docs: t-elem ::= WSP / chord-or-text / gracing / grace-notes / broken-rhythm
  / slur-begin / slur-end