code: Rule FieldNumber() {
  		return Sequence(
  				ZeroOrMoreS(FirstOfS(Eols().suppressNode(),
  								Comment(), Xcommand(), Tex())),
  				String("X:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				DIGITS(),
  				HeaderEol()
  			).label(FieldNumber);
  	}
docs: field-number ::= %x58.3A *WSP 1*DIGIT header-eol<p>
  <tt>X:</tt>