docs: public Set<String> getCountryCodes() {
          Set<String> countryCodes = new HashSet<String>();
          for (Entry<String, CurrencyUnit> entry : currenciesByCountry.entrySet()) {
              if (this.equals(entry.getValue())) {
                  countryCodes.add(entry.getKey());
              }
          }
          return countryCodes;
      }