@Test
public void testSumOfAFewLongs() throws Throwable {

    Observable<Long> src = Observable.from(1L, 2L, 3L, 4L, 5L);
    sumLongs(src).subscribe(wl);
    verify(wl, times(1)).onNext(anyLong());
    verify(wl).onNext(15L);
    verify(wl, never()).onError(any(Throwable.class));
    verify(wl, times(1)).onCompleted();
}