code: public PeriodFormatterBuilder appendSeparator(String text) {
          return appendSeparator(text, text, null, true, true);
      }
docs: Append a separator, which is output if fields are printed both before
  and after the separator.
  <p>
  For example, <code>builder.appendDays().appendSeparator(",").appendHours()</code>
  will only output the comma if both the days and hours fields are output.
  <p>
  The text will be parsed case-insensitively.
  <p>
  Note: appending a separator discontinues any further work on the latest
  appended field.
  
  @param text  the text to use as a separator
  @return this PeriodFormatterBuilder
  @throws IllegalStateException if this separator follows a previous one