public static <T, K> AggregateOperation1<T, Map<K, List<T>>, Map<K, List<T>>> groupingBy(
            FunctionEx<? super T, ? extends K> keyFn
    ) {
        checkSerializable(keyFn, "keyFn");
        return groupingBy(keyFn, toList());
    }
@Test
    public void when_groupingBy_withSameKey() {
        Entry<String, Integer> entryA = entry("a", 1);
        validateOpWithoutDeduct(
                groupingBy(entryKey()),
                identity(),
                entryA,
                entryA,
                asMap("a", singletonList(entryA)),
                asMap("a", asList(entryA, entryA)),
                asMap("a", asList(entryA, entryA))
        );
    }