code: private MoneyFormatterBuilder appendInternal(MoneyPrinter printer, MoneyParser parser) {
          printers.add(printer);
          parsers.add(parser);
          return this;
      }
docs: Appends the specified printer and parser to this builder.
  <p>
  Either the printer or parser must be non-null.
  
  @param printer  the printer to append, null makes the formatter unable to print
  @param parser  the parser to append, null makes the formatter unable to parse
  @return this for chaining, never null