code: Rule FieldPart() {
  		return Sequence(
  				String("P:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				FirstOf(CharRange('A', 'Z'),
  						CharRange('a', 'z')).label("ALPHA").suppressSubnodes(),
  				OptionalS(TexText()),
  				HeaderEol())
  			.label(FieldPart);
  	}
docs: field-part ::= %x50.3A *WSP (ALPHA / tex-text) header-eol
  <p><tt>P:</tt>