public Optional<Model> getPreviousModel() { return previousModel; }
@Test
    void testPreviousModelIsProvided() throws IOException, SAXException {
        VespaModel prevModel = new VespaModel(MockApplicationPackage.createEmpty());
        DeployState.Builder builder = new DeployState.Builder();
        assertEquals(prevModel, builder.previousModel(prevModel).build().getPreviousModel().get());
    }