@Test
public void testEmptySumLongs() throws Throwable {

    Observable<Long> src = Observable.empty();
    sumLongs(src).subscribe(wl);
    verify(wl, times(1)).onNext(anyLong());
    verify(wl).onNext(0L);
    verify(wl, never()).onError(any(Throwable.class));
    verify(wl, times(1)).onCompleted();
}