code: private void addProvider(ExchangeRateProvider prov) {
          providers.add(Optional.ofNullable(prov)
                  .orElseThrow(() -> new NullPointerException("ConversionProvider required.")));
      }
docs: Add an additional {@link ExchangeRateProvider} to the instance's delegate
  list.
  
  @param prov The {@link ExchangeRateProvider} to be added, not {@code null}
  .
  @throws java.lang.NullPointerException if the provider is null.
  .