@CheckForNull
  public Entry<K, V> lowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }

  @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) {

  @CheckForNull
  public Entry<K, V> lowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }

  @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) {

      synchronized (mutex) {
        return delegate().higherKey(key);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().lastEntry(), mutex);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lowerEntry(K key) {
      synchronized (mutex) {

      synchronized (mutex) {
        return delegate().higherKey(key);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().lastEntry(), mutex);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lowerEntry(K key) {
      synchronized (mutex) {