code: Rule ParenVoice() {
  		return Sequence('(', SingleVoice(),
  				OneOrMore(Sequence(OneOrMore(WSP()), SingleVoice())))
  			.label(ParenVoice);
  	}
docs: paren-voice ::= "(" single-voice 1*( 1*WSP single-voice) ")"
  <p>
  on same staff