code: @FromString
      public static Seconds parseSeconds(String periodStr) {
          if (periodStr == null) {
              return Seconds.ZERO;
          }
          Period p = PARSER.parsePeriod(periodStr);
          return Seconds.seconds(p.getSeconds());
      }
docs: Creates a new <code>Seconds</code> by parsing a string in the ISO8601 format 'PTnS'.
  <p>
  The parse will accept the full ISO syntax of PnYnMnWnDTnHnMnS however only the
  seconds component may be non-zero. If any other component is non-zero, an exception
  will be thrown.
  
  @param periodStr  the period string, null returns zero
  @return the period in seconds
  @throws IllegalArgumentException if the string format is invalid