}

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

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

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

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

      return map().higherKey(e);
    }

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

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

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

      return map().higherKey(e);
    }

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

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

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