code: Rule GraceNotes() {
  		return Sequence(
  				String("{"),
  				OptionalS(Acciaccatura()),
  				ZeroOrMoreS(GraceNoteStem()),
  				String("}"))
  		.label(GraceNotes);
  	}
docs: grace notes can have length
  
  grace-notes ::= "{" acciaccatura 1*( grace-note-stem ) "}"