}

  public void testHeadMap_k() {
    NavigableMap<String, Integer> map = create();
    SortedMap<String, Integer> headMap = map.headMap("a");
    assertTrue(headMap instanceof SynchronizedSortedMap);
    assertThat(((SynchronizedSortedMap<String, Integer>) headMap).mutex).isSameInstanceAs(mutex);
  }

  public void testHeadMap_k_b() {
    NavigableMap<String, Integer> map = create();