code: Rule IfieldGroup() {
  		return Sequence(String("[G:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				TexTextIfield(),
  				String("]"))
  			.label(IfieldGroup);
  	}
docs: ifield-group ::= %5B.%47.%3A *WSP tex-text-ifield %5D<p>
  <tt>[G:..]</tt>