assertTrue(subMap instanceof SynchronizedSortedMap);
    assertThat(((SynchronizedSortedMap<String, Integer>) subMap).mutex).isSameInstanceAs(mutex);
  }

  public void testSubMap_k_b_k_b() {
    NavigableMap<String, Integer> map = create();
    NavigableMap<String, Integer> subMap = map.subMap("a", true, "b", false);
    assertTrue(subMap instanceof SynchronizedNavigableMap);