ImmutableSortedSet<String> sortedSet = zis.combine(zat).build();
    assertThat(sortedSet).containsExactly("a", "b", "c", "d").inOrder();
  }

  // TODO(b/172823566): Use mainline testToImmutableSortedSet_customComparator once CollectorTester
  //  is usable to java7.
  public void testToImmutableSortedSet_customComparator_java7() {
    // Note that a Collector should generally enforce consistent comparator between builders.

    CollectorTester.of(collector, equivalence)
        .expectCollects(
            ImmutableSortedSet.of("a", "b", "c", "d"), "a", "b", "a", "c", "b", "b", "d");
  }

  public void testToImmutableSortedSet_customComparator() {
    Collector<String, ?, ImmutableSortedSet<String>> collector =
        toImmutableSortedSet(String.CASE_INSENSITIVE_ORDER);