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();
    }
@Test
    public void testPaymentAck() throws Exception {
        // Create
        Payment paymentMessage = Protos.Payment.newBuilder().build();
        PaymentACK paymentAck = PaymentProtocol.createPaymentAck(paymentMessage, MEMO);
        byte[] paymentAckBytes = paymentAck.toByteArray();

        // Parse
        PaymentACK parsedPaymentAck = PaymentACK.parseFrom(paymentAckBytes);
        assertEquals(paymentMessage, parsedPaymentAck.getPayment());
        assertEquals(MEMO, parsedPaymentAck.getMemo());
    }