}

  public void testNavigableKeySet() {
    NavigableMap<String, Integer> map = create();
    NavigableSet<String> navigableKeySet = map.navigableKeySet();
    assertTrue(navigableKeySet instanceof SynchronizedNavigableSet);
    assertSame(mutex, ((SynchronizedNavigableSet<String>) navigableKeySet).mutex);
  }

  public void testPollFirstEntry() {
    create().pollFirstEntry();
  }

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

    @LazyInit @CheckForNull private transient NavigableSet<K> navigableKeySet;

    @Override
    public NavigableSet<K> navigableKeySet() {
      NavigableSet<K> result = navigableKeySet;
      return (result == null) ? navigableKeySet = new NavigableKeySet<>(this) : result;
    }

    @Override

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

    @LazyInit @CheckForNull private transient NavigableSet<K> navigableKeySet;

    @Override
    public NavigableSet<K> navigableKeySet() {
      NavigableSet<K> result = navigableKeySet;
      return (result == null) ? navigableKeySet = new NavigableKeySet<>(this) : result;
    }

    @Override

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

    @CheckForNull transient NavigableSet<K> navigableKeySet;

    @Override
    public NavigableSet<K> navigableKeySet() {
      synchronized (mutex) {
        if (navigableKeySet == null) {
          return navigableKeySet = Synchronized.navigableSet(delegate().navigableKeySet(), mutex);
        }
        return navigableKeySet;

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

    @CheckForNull transient NavigableSet<K> navigableKeySet;

    @Override
    public NavigableSet<K> navigableKeySet() {
      synchronized (mutex) {
        if (navigableKeySet == null) {
          return navigableKeySet = Synchronized.navigableSet(delegate().navigableKeySet(), mutex);
        }
        return navigableKeySet;

            (RegularImmutableSortedSet<K>) keySet.descendingSet(), valueList.reverse(), this);
      }
    }
    return result;
  }

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

  @Override
  public ImmutableSortedSet<K> descendingKeySet() {
    return keySet.descendingSet();
  }

  /**

            (RegularImmutableSortedSet<K>) keySet.descendingSet(), valueList.reverse(), this);
      }
    }
    return result;
  }

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

  @Override
  public ImmutableSortedSet<K> descendingKeySet() {
    return keySet.descendingSet();
  }

  /**