docs: public static Protos.Payment createPaymentMessage(List<Transaction> transactions,
              @Nullable Coin refundAmount, @Nullable Address refundAddress, @Nullable String memo,
              @Nullable byte[] merchantData) {
          if (refundAddress != null) {
              if (refundAmount == null)
                  throw new IllegalArgumentException("Specify refund amount if refund address is specified.");
              return createPaymentMessage(transactions,
                      ImmutableList.of(createPayToAddressOutput(refundAmount, refundAddress)), memo, merchantData);
          } else {
              return createPaymentMessage(transactions, null, memo, merchantData);
          }
      }