TDB versus an OntModel

here's a little test method. The second call to smallAssertions sees
no triples at all. There's a todo on the doc for ontologies, and I'll
cheerfully write some doc if someone will tell me what dumb thing I've
missed here.

@Test
public void tdb() throws Exception {
Dataset dataset =
TDBFactory.createDataset(tempDataset.getAbsolutePath());
dataset.begin(ReadWrite.WRITE);
Model model = dataset.getDefaultModel();
OntModel ontModel =
ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM, model);
ResRdfBuilder resRdfBuilder = new ResRdfBuilder(ontModel, 1);
ResTsvReader reader = new ResTsvReader();
URL dataUrl = Resources.getResource(ResTsvReaderTest.class, "data.txt");
ByteSource dataSource = Resources.asByteSource(dataUrl);
reader.read(dataSource, resRdfBuilder);
smallAssertions(ontModel);
dataset.end();

dataset.begin(ReadWrite.READ);
model = dataset.getDefaultModel();
smallAssertions(model);
dataset.end();