static int nextId;
    Integer id = nextId++;

    @Override
    public int compareTo(Interface other) {
      return id.compareTo(((Impl) other).id);
    }
  }

  public void testOf_ordering_dupes() {
    SortedSet<String> set = of("e", "a", "e", "f", "b", "b", "d", "a", "c");
    assertThat(set).containsExactly("a", "b", "c", "d", "e", "f").inOrder();
  }

  public void testOf_comparator() {