@Override
    @CheckForNull
    public Comparator<? super K> comparator() {
      return fromMap().comparator();
    }

    @Override
    @ParametricNullness
    public K firstKey() {
      return fromMap().firstKey();
    }

    @Override
    public SortedMap<K, V2> headMap(@ParametricNullness K toKey) {
      return transformEntries(fromMap().headMap(toKey), transformer);
    }