assertEquals(SORTED_EMPTY, diff.entriesInCommon());
    assertEquals(SORTED_EMPTY, diff.entriesDiffering());
    assertThat(diff.toString()).isEqualTo("not equal: only on left={1=2}");
  }

  public void testSortedMapDifferenceTypical() {
    SortedMap<Integer, String> left =
        ImmutableSortedMap.<Integer, String>reverseOrder()
            .put(1, "a")
            .put(2, "b")
            .put(3, "c")
            .put(4, "d")