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

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

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

  public void testLastEntry() {
    create().lastEntry();
  }