@Test
public void testEmptySumDoubles() throws Throwable {

    Observable<Double> src = Observable.empty();
    sumDoubles(src).subscribe(wd);
    verify(wd, times(1)).onNext(anyDouble());
    verify(wd).onNext(0.0d);
    verify(wd, never()).onError(any(Throwable.class));
    verify(wd, times(1)).onCompleted();
}