return transformEntries(fromMap().headMap(toKey), transformer);
    }

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

    @Override
    public SortedMap<K, V2> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {

      return transformEntries(fromMap().headMap(toKey), transformer);
    }

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

    @Override
    public SortedMap<K, V2> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {

            If not specified, it defaults to {@code ../pom.xml}.
            Maven looks for the parent POM first in this location on
            the filesystem, then the local repository, and lastly in the remote repo.
            {@code relativePath} allows you to select a different location,
            for example when your structure is flat, or deeper without an intermediate parent POM.