assertTrue(headMap instanceof SynchronizedNavigableMap);
    assertThat(((SynchronizedNavigableMap<String, Integer>) headMap).mutex).isSameInstanceAs(mutex);
  }

  public void testHigherEntry() {
    assertThat(create().higherEntry("a")).isNull();
  }

  public void testHigherKey() {
    assertThat(create().higherKey("a")).isNull();
  }

  public void testLastEntry() {