code: Rule IfieldSource() {
  		return Sequence(String("[S:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				TexTextIfield(),
  				String("]"))
  			.label(IfieldSource);
  	}
docs: ifield-source ::= %5B.%53.%3A *WSP tex-text-ifield %5D<p>
  <tt>[S:..]</tt>