code: public static CurrencyUnit registerCurrencyUnit(CurrencyUnit currencyUnit, Locale locale) {
          Objects.requireNonNull(locale);
          Objects.requireNonNull(currencyUnit);
          return ConfigurableCurrencyUnitProvider.currencyUnitsByLocale.put(locale, currencyUnit);
      }
docs: Registers a bew currency unit under the given Locale.
  
  @param currencyUnit the new currency to be registered, not null.
  @param locale       the Locale, not null.
  @return any unit instance registered previously by this instance, or null.