@Override
  @CheckForNull
  public K lowerKey(K key) {
    return keyOrNull(lowerEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override
  @CheckForNull
  public K floorKey(K key) {
    return keyOrNull(floorEntry(key));
  }

  @Override
  @CheckForNull

  @Override
  @CheckForNull
  public K lowerKey(K key) {
    return keyOrNull(lowerEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override
  @CheckForNull
  public K floorKey(K key) {
    return keyOrNull(floorEntry(key));
  }

  @Override
  @CheckForNull

      return map().higherKey(e);
    }

    @Override
    @CheckForNull
    public K pollFirst() {
      return keyOrNull(map().pollFirstEntry());
    }

    @Override
    @CheckForNull
    public K pollLast() {
      return keyOrNull(map().pollLastEntry());
    }

    @Override
    public NavigableSet<K> descendingSet() {
      return map().descendingKeySet();

      return map().higherKey(e);
    }

    @Override
    @CheckForNull
    public K pollFirst() {
      return keyOrNull(map().pollFirstEntry());
    }

    @Override
    @CheckForNull
    public K pollLast() {
      return keyOrNull(map().pollLastEntry());
    }

    @Override
    public NavigableSet<K> descendingSet() {
      return map().descendingKeySet();