code: Rule IfieldRhythm() {
  		return Sequence(String("[R:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				TexTextIfield(),
  				String("]"))
  			.label(IfieldRhythm);
  	}
docs: ifield-rhythm ::= %5B.%52.%3A *WSP tex-text-ifield %5D<p>
  <tt>[R:reel]</tt>