@Test
public void testThatYouCannotBuyIfThereIsNotEnoughShare(TestContext tc) {

  Async async = tc.async();
  service.buy(100, quote("A", 10, 20, 10), ar -> {
    tc.assertTrue(ar.failed());
    tc.assertTrue(ar.cause().getMessage().contains("not enough stocks"));
    async.complete();
  });
}