assertThrows(
        IllegalArgumentException.class,
        () -> Stream.of(mapEntry("one", 1), mapEntry("one", 11)).collect(collector));
  }

  public void testToImmutableSortedMapMerging() {
    Collector<Entry<String, Integer>, ?, ImmutableSortedMap<String, Integer>> collector =
        toImmutableSortedMap(naturalOrder(), Entry::getKey, Entry::getValue, Integer::sum);