code: Rule IfieldOrigin() {
  		return Sequence(String("[O:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				TexTextIfield(),
  				String("]"))
  			.label(IfieldOrigin);
  	}
docs: ifield-origin ::= %5B.%4F.%3A *WSP tex-text-ifield %5D<p>
  <tt>[O:..]</tt>