code: Rule IUnusedIfield() {
  		return Sequence(String("["),
  				AnyOf("EIJYabcdefghijklnopqrstvxyz"),
  				String(":"),
  				ZeroOrMore(FirstOf(WSP(), VCHAR())),
  				String("]"))
  			.label(IUnusedIfield);
  	}
docs: E: I: J: Y: a:-l: n:-t: v: x: y: z: ignore - but for backward and forward compatibility
  <p>
  unused-ifield ::= %5B (%x45 / %x49 / %4A / %59 / %61-6c / %6e-%74 / %76 / %78-7A)
  %3A *(WSP / VCHAR) %5D