}

  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();

      SortedMap<K, V> sortedMap, @Nullable Object mutex) {
    return new SynchronizedSortedMap<>(sortedMap, mutex);
  }

  static class SynchronizedSortedMap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMap<K, V> implements SortedMap<K, V> {

    SynchronizedSortedMap(SortedMap<K, V> delegate, @Nullable Object mutex) {
      super(delegate, mutex);
    }

      SortedMap<K, V> sortedMap, @Nullable Object mutex) {
    return new SynchronizedSortedMap<>(sortedMap, mutex);
  }

  static class SynchronizedSortedMap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMap<K, V> implements SortedMap<K, V> {

    SynchronizedSortedMap(SortedMap<K, V> delegate, @Nullable Object mutex) {
      super(delegate, mutex);
    }