@Override
  protected <K, V> NavigableMap<K, V> create() {
    @SuppressWarnings("unchecked")
    NavigableMap<K, V> innermost =
        new SafeTreeMap<>((Comparator<? super K>) Ordering.natural().nullsFirst());
    TestMap<K, V> inner = new TestMap<>(innermost, mutex);
    NavigableMap<K, V> outer = Synchronized.navigableMap(inner, mutex);
    return outer;
  }

     * restriction.
     */
    private final Range<C> restriction;

    private final NavigableMap<Cut<C>, Range<C>> rangesByLowerBound;
    private final NavigableMap<Cut<C>, Range<C>> rangesByUpperBound;

    private SubRangeSetRangesByLowerBound(
        Range<Cut<C>> lowerBoundWindow,
        Range<C> restriction,
        NavigableMap<Cut<C>, Range<C>> rangesByLowerBound) {
      this.lowerBoundWindow = checkNotNull(lowerBoundWindow);

  }

  <K, V> void testNavigationAgainstExpected(
      NavigableMap<K, V> expected, NavigableMap<K, V> navigableMap, Iterable<K> keysToTest) {
    for (K key : keysToTest) {
      assertEquals(expected.lowerEntry(key), navigableMap.lowerEntry(key));
      assertEquals(expected.floorEntry(key), navigableMap.floorEntry(key));
      assertEquals(expected.ceilingEntry(key), navigableMap.ceilingEntry(key));

     * restriction.
     */
    private final Range<C> restriction;

    private final NavigableMap<Cut<C>, Range<C>> rangesByLowerBound;
    private final NavigableMap<Cut<C>, Range<C>> rangesByUpperBound;

    private SubRangeSetRangesByLowerBound(
        Range<Cut<C>> lowerBoundWindow,
        Range<C> restriction,
        NavigableMap<Cut<C>, Range<C>> rangesByLowerBound) {
      this.lowerBoundWindow = checkNotNull(lowerBoundWindow);

    return Iterators.pollNext(descendingMap().entrySet().iterator());
  }

  @Override
  public NavigableMap<K, V> descendingMap() {
    return delegate().descendingMap();
  }

  /**
   * A sensible implementation of {@link NavigableMap#descendingMap} in terms of the methods of this
   * {@code NavigableMap}. In many cases, you may wish to override {@link

    return Iterators.pollNext(descendingMap().entrySet().iterator());
  }

  @Override
  public NavigableMap<K, V> descendingMap() {
    return delegate().descendingMap();
  }

  /**
   * A sensible implementation of {@link NavigableMap#descendingMap} in terms of the methods of this
   * {@code NavigableMap}. In many cases, you may wish to override {@link

    suite.addTest(
        NavigableMapTestSuiteBuilder.using(
                new TestStringSortedMapGenerator() {
                  @Override
                  protected NavigableMap<String, String> create(Entry<String, String>[] entries) {
                    NavigableMap<String, String> map = new SafeTreeMap<>();
                    putEntries(map, entries);
                    map.put("banana", "toast");

    }
  }

  private final class NavigableAsMap extends SortedAsMap implements NavigableMap<K, Collection<V>> {

    NavigableAsMap(NavigableMap<K, Collection<V>> submap) {
      super(submap);
    }

    @Override
    NavigableMap<K, Collection<V>> sortedMap() {
      return (NavigableMap<K, Collection<V>>) super.sortedMap();
    }

    @Override
    @CheckForNull

    return SortedMapTestSuiteBuilder.using(
            new TestStringSortedMapGenerator() {
              @Override
              protected NavigableMap<String, String> create(Entry<String, String>[] entries) {
                NavigableMap<String, String> map = populate(new TreeMap<String, String>(), entries);
                return Collections.checkedNavigableMap(map, String.class, String.class);
              }

    }
  }

  private final class NavigableAsMap extends SortedAsMap implements NavigableMap<K, Collection<V>> {

    NavigableAsMap(NavigableMap<K, Collection<V>> submap) {
      super(submap);
    }

    @Override
    NavigableMap<K, Collection<V>> sortedMap() {
      return (NavigableMap<K, Collection<V>>) super.sortedMap();
    }

    @Override
    @CheckForNull