@Test
public void testEmptySum() throws Throwable {

    Observable<Integer> src = Observable.empty();
    sum(src).subscribe(w);
    verify(w, times(1)).onNext(anyInt());
    verify(w).onNext(0);
    verify(w, never()).onError(any(Throwable.class));
    verify(w, times(1)).onCompleted();
}