@Test
public void onSubscribe() throws Exception {

    List<Throwable> errors = TestHelper.trackPluginErrors();
    try {
        Disposable s = Disposables.empty();
        fo.onSubscribe(s);
        Disposable s2 = Disposables.empty();
        fo.onSubscribe(s2);
        assertFalse(s.isDisposed());
        assertTrue(s2.isDisposed());
        TestHelper.assertError(errors, 0, IllegalStateException.class, "Disposable already set!");
    } finally {
        RxJavaPlugins.reset();
    }
}