@Test
public void testSumOfAFewInts() throws Throwable {

    Observable<Integer> src = Observable.from(1, 2, 3, 4, 5);
    sum(src).subscribe(w);
    verify(w, times(1)).onNext(anyInt());
    verify(w).onNext(15);
    verify(w, never()).onError(any(Throwable.class));
    verify(w, times(1)).onCompleted();
}