docs: public BigMoney plus(BigDecimal amountToAdd) {
          MoneyUtils.checkNotNull(amountToAdd, "Amount must not be null");
          if (amountToAdd.compareTo(BigDecimal.ZERO) == 0) {
              return this;
          }
          BigDecimal newAmount = amount.add(amountToAdd);
          return BigMoney.of(currency, newAmount);
      }