code: public RoundingContextBuilder setCurrency(CurrencyUnit currencyUnit) {
          Objects.requireNonNull(currencyUnit);
          return set(CurrencyUnit.class, currencyUnit);
      }
docs: Get the basic {@link CurrencyUnit}, which is based for this rounding type.
  @param currencyUnit the target currency unit not null.
  @return the target CurrencyUnit, or null.