code: Rule GraceNoteStem() {
  		return FirstOf(
  			GraceNote(),
  			Sequence(suppr("["),
  					GraceNote(),
  					OneOrMoreS(GraceNote()),
  					suppr("]")).label(GraceMultiNote)
  		).label(GraceNoteStem);
  	}
docs: grace-note-stem ::= grace-note / ( "[" 2*grace-note "]" )