code: public PeriodFormatterBuilder append(PeriodPrinter printer, PeriodParser parser) {
          if (printer == null && parser == null) {
              throw new IllegalArgumentException("No printer or parser supplied");
          }
          clearPrefix();
          append0(printer, parser);
          return this;
      }
docs: Appends a printer parser pair.
  <p>
  Either the printer or the parser may be null, in which case the builder will
  be unable to produce a parser or printer respectively.
  
  @param printer  appends a printer to the builder, null if printing is not supported
  @param parser  appends a parser to the builder, null if parsing is not supported
  @return this PeriodFormatterBuilder
  @throws IllegalArgumentException if both the printer and parser are null