code: public PeriodFormatterBuilder appendPrefix(String singularText,
                                                   String pluralText) {
          if (singularText == null || pluralText == null) {
              throw new IllegalArgumentException();
          }
          return appendPrefix(new PluralAffix(singularText, pluralText));
      }
docs: Append a field prefix which applies only to the next appended field. If
  the field is not printed, neither is the prefix.
  <p>
  During parsing, the singular and plural versions are accepted whether
  or not the actual value matches plurality.
  
  @param singularText text to print if field value is one
  @param pluralText text to print if field value is not one
  @return this PeriodFormatterBuilder
  @see #appendSuffix