* {@link #subMap(Object, Object)} to forward to this implementation.
   *
   * @since 7.0
   */
  protected SortedMap<K, V> standardSubMap(K fromKey, K toKey) {
    checkArgument(unsafeCompare(comparator(), fromKey, toKey) <= 0, "fromKey must be <= toKey");
    return tailMap(fromKey).headMap(toKey);
  }

   * {@link #subMap(Object, Object)} to forward to this implementation.
   *
   * @since 7.0
   */
  protected SortedMap<K, V> standardSubMap(K fromKey, K toKey) {
    checkArgument(unsafeCompare(comparator(), fromKey, toKey) <= 0, "fromKey must be <= toKey");
    return tailMap(fromKey).headMap(toKey);
  }

    }

    @Override
    public boolean isEmpty() {
      return standardIsEmpty();
    }

    @Override
    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      return standardSubMap(fromKey, toKey);
    }

    @Override
    public @Nullable Entry<K, V> lowerEntry(K key) {
      return standardLowerEntry(key);
    }

    @Override
    public @Nullable K lowerKey(K key) {