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

  public void testToImmutableSet_duplicates() {
    class TypeWithDuplicates {
      final int a;
      final int b;

      TypeWithDuplicates(int a, int b) {
        this.a = a;
        this.b = b;
      }

      @Override