code: Rule IfieldPart() {
  		return Sequence(String("[P:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				FirstOfS(CharRange('A', 'Z').label("ALPHA"), TexTextIfield()),
  				String("]"))
  			.label(IfieldPart);
  	}
docs: ifield-part ::= %5B.%50.%3A *WSP (ALPHA / tex-text-ifield) %5D<p>
  <tt>[P:A]</tt>