code: Rule IfieldKey() {
  		return Sequence(String("[K:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				Key(),
  				String("]"))
  			.label(IfieldKey);
  	}
docs: ifield-key ::= %5B.4B.3A *WSP key %5D<p>
  e.g. <tt>[K:Ador] [K: Bphr ^d]</tt>