}

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

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

  }

  @Override
  public @Nullable K lowerKey(K key) {
    return delegate.lowerKey(checkValid(key));
  }

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

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return delegate.pollFirstEntry();
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {

  }

  @Override
  public @Nullable K lowerKey(K key) {
    return delegate.lowerKey(checkValid(key));
  }

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

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return delegate.pollFirstEntry();
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {

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

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

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

    @Override