code: private PeriodFormatterBuilder appendPrefix(PeriodFieldAffix prefix) {
          if (prefix == null) {
              throw new IllegalArgumentException();
          }
          if (iPrefix != null) {
              prefix = new CompositeAffix(iPrefix, prefix);
          }
          iPrefix = prefix;
          return this;
      }
docs: Append a field prefix which applies only to the next appended field. If
  the field is not printed, neither is the prefix.
  
  @param prefix custom prefix
  @return this PeriodFormatterBuilder
  @see #appendSuffix