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

  public void testToImmutableMapMerging() {
    Collector<Entry<String, Integer>, ?, ImmutableMap<String, Integer>> collector =
        toImmutableMap(Entry::getKey, Entry::getValue, Integer::sum);
    Equivalence<ImmutableMap<String, Integer>> equivalence =