docs: public static Protos.PaymentACK createPaymentAck(Protos.Payment paymentMessage, @Nullable String memo) {
          final Protos.PaymentACK.Builder builder = Protos.PaymentACK.newBuilder();
          builder.setPayment(paymentMessage);
          if (memo != null)
              builder.setMemo(memo);
          return builder.build();
      }