NavigableMap<String, Integer> map = create();
    NavigableSet<String> descendingKeySet = map.descendingKeySet();
    assertTrue(descendingKeySet instanceof SynchronizedNavigableSet);
    assertSame(mutex, ((SynchronizedNavigableSet<String>) descendingKeySet).mutex);
  }

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

                NavigableSet<String> delegate = new TreeSet<>(MinimalCollection.of(elements));
                return Collections.synchronizedNavigableSet(delegate);
              }
            })
        .named("synchronizedNavigableSet/TreeSet, natural")
        .withFeatures(
            SetFeature.GENERAL_PURPOSE,
            CollectionFeature.SERIALIZABLE,