code: public TimeZone cloneAsThawed() {
          RuleBasedTimeZone tz = (RuleBasedTimeZone)super.cloneAsThawed();
          if (historicRules != null) {
              tz.historicRules = new ArrayList<TimeZoneRule>(historicRules); // rules are immutable
          }
          if (finalRules != null) {
              tz.finalRules = finalRules.clone();
          }
          tz.isFrozen = false;
          return tz;
      }
docs: {@inheritDoc}