src/main/java/org/organicdesign/fp/collections/PersistentTreeSet.java
@Override public ImSortedSet<E> subSet(E fromElement, E toElement) {
        return PersistentTreeSet.ofMap(impl.subMap(fromElement, toElement));
    }
@Override public ImSortedSet<E> subSet(E fromElement, E toElement) {
        return PersistentTreeSet.ofMap(impl.subMap(fromElement, toElement));
    }
{@inheritDoc}