docs: public static Protos.Payment createPaymentMessage(List<Transaction> transactions,
              @Nullable List<Protos.Output> refundOutputs, @Nullable String memo, @Nullable byte[] merchantData) {
          Protos.Payment.Builder builder = Protos.Payment.newBuilder();
          for (Transaction transaction : transactions) {
              transaction.verify();
              builder.addTransactions(ByteString.copyFrom(transaction.unsafeBitcoinSerialize()));
          }
          if (refundOutputs != null) {
              for (Protos.Output output : refundOutputs)
                  builder.addRefundTo(output);
          }
          if (memo != null)
              builder.setMemo(memo);
          if (merchantData != null)
              builder.setMerchantData(ByteString.copyFrom(merchantData));
          return builder.build();
      }