code: public PeriodFormatterBuilder appendPrefix(String text) {
          if (text == null) {
              throw new IllegalArgumentException();
          }
          return appendPrefix(new SimpleAffix(text));
      }
docs: Append a field prefix which applies only to the next appended field. If
  the field is not printed, neither is the prefix.
  
  @param text text to print before field only if field is printed
  @return this PeriodFormatterBuilder
  @see #appendSuffix