}

  public void testDescendingKeySet() {
    NavigableMap<String, Integer> map = create();
    NavigableSet<String> descendingKeySet = map.descendingKeySet();
    assertTrue(descendingKeySet instanceof SynchronizedNavigableSet);
    assertThat(((SynchronizedNavigableSet<String>) descendingKeySet).mutex).isSameInstanceAs(mutex);
  }

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

    public StandardNavigableKeySet() {
      super(ForwardingNavigableMap.this);
    }
  }

  @Override
  public NavigableSet<K> descendingKeySet() {
    return delegate().descendingKeySet();
  }

  /**
   * A sensible definition of {@link #descendingKeySet} as the {@code navigableKeySet} of {@link
   * #descendingMap}. (The {@link StandardDescendingMap} implementation implements {@code

    public StandardNavigableKeySet() {
      super(ForwardingNavigableMap.this);
    }
  }

  @Override
  public NavigableSet<K> descendingKeySet() {
    return delegate().descendingKeySet();
  }

  /**
   * A sensible definition of {@link #descendingKeySet} as the {@code navigableKeySet} of {@link
   * #descendingMap}. (The {@link StandardDescendingMap} implementation implements {@code

      return new NavigableKeySet(sortedMap());
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return keySet();
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
      return descendingMap().navigableKeySet();
    }

    @Override
    public NavigableMap<K, Collection<V>> subMap(
        @ParametricNullness K fromKey, @ParametricNullness K toKey) {

      return new NavigableKeySet(sortedMap());
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return keySet();
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
      return descendingMap().navigableKeySet();
    }

    @Override
    public NavigableMap<K, Collection<V>> subMap(
        @ParametricNullness K fromKey, @ParametricNullness K toKey) {