code: Rule FieldUserdefPrint() {
  		return Sequence(String("U:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				Userdef(), HeaderEol()).label(FieldUserdefPrint);
  	}
docs: field-userdef-print ::= %x55.3A *WSP userdef header-eol<p>
  <tt>U:</tt>