public static <T> AggregateOperation1<T, Set<T>, Set<T>> toSet() {
        return toCollection(HashSet::new);
    }
@Test
    public void when_toSet() {
        validateOpWithoutDeduct(
                toSet(), identity(), 1, 2, singleton(1), new HashSet<>(asList(1, 2)), new HashSet<>(asList(1, 2)));
    }