docs: public static ISOChronology getInstance(DateTimeZone zone) {
          if (zone == null) {
              zone = DateTimeZone.getDefault();
          }
          ISOChronology chrono = cCache.get(zone);
          if (chrono == null) {
              chrono = new ISOChronology(ZonedChronology.getInstance(INSTANCE_UTC, zone));
              ISOChronology oldChrono = cCache.putIfAbsent(zone, chrono);
              if (oldChrono != null) {
                  chrono = oldChrono;
              }
          }
          return chrono;
      }