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

  @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) {

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

  @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) {

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

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

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

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

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

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

     * @param map the sequenced map
     * @return the last entry, or null if the map is empty
     * @since Java 21
     */
    public static <K, V> Map.Entry<K, V> lastEntry(final SequencedMap<K, V> map) {
        return map.isEmpty() ? null : map.lastEntry();
    }

    /**
     * Returns a reversed view of a sequenced map.
     *
     * @param <K> the key type
     * @param <V> the value type

    {
      Entry<Integer, String> entry = unmod.firstEntry();
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      Entry<Integer, String> entry = unmod.lastEntry();
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      @SuppressWarnings("unchecked")