code: Rule IfieldVoice() {
  		return Sequence(String("[V:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				Voice(),
  				String("]"))
  			.label(IfieldVoice);
  	}
docs: ifield-voice ::= %5B.%56.%3A *WSP voice %5D<p>
  <tt>[V:..]</tt> options should be better defined