code: Rule IfieldLength() {
  		return Sequence(String("[L:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				NoteLengthStrict(),
  				String("]"))
  			.label(IfieldLength);
  	}
docs: ifield-length ::= %5B.4C.3A *WSP note-length-strict %5D<p>
  e.g. <tt>[L:1/8]</tt>