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