code: 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;
      }
docs: Gets the country codes applicable to this currency.
  <p>
  A currency is typically valid in one or more countries.
  The codes are typically defined by ISO-3166.
  An empty set indicates that no the currency is not associated with a country code.
  
  @return the country codes, may be empty, not null