@Override
    public NavigableMap<K, V2> subMap(
        @ParametricNullness K fromKey,
        boolean fromInclusive,
        @ParametricNullness K toKey,
        boolean toInclusive) {
      return transformEntries(
          fromMap().subMap(fromKey, fromInclusive, toKey, toInclusive), transformer);
    }

    @Override