return Maps.keyOrNull(higherEntry(key));
  }

  abstract Iterator<Entry<K, V>> descendingEntryIterator();

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

  @Override
  public SortedMap<K, V> headMap(@ParametricNullness K toKey) {
    return headMap(toKey, false);
  }

  @Override

    return Maps.keyOrNull(higherEntry(key));
  }

  abstract Iterator<Entry<K, V>> descendingEntryIterator();

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

  @Override
  public SortedMap<K, V> headMap(@ParametricNullness K toKey) {
    return headMap(toKey, false);
  }

  @Override