persistable static class SynchronizedSortedSet<E>
    extends SynchronizedSet<E>
    implements SortedSet<E>
{
    @java.io.Serial
    private static final long serialVersionUID = 8695801310862127406L;

    @SuppressWarnings("serial") // Conditionally serializable
    /*private final*/ SortedSet<E> ss;

    SynchronizedSortedSet(SortedSet<E> s) {
        super(s);
        ss = s;
    }
    SynchronizedSortedSet(SortedSet<E> s, Object mutex) {
        super(s, mutex);
        ss = s;
    }

    public Comparator<? super E> comparator() {
        synchronized (mutex) {return ss.comparator();}
    }

    public SortedSet<E> subSet(E fromElement, E toElement) {
        synchronized (mutex) {
            return new SynchronizedSortedSet<>(
                ss.subSet(fromElement, toElement), mutex);
        }
    }
    public SortedSet<E> headSet(E toElement) {
        synchronized (mutex) {
            return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);
        }
    }
    public SortedSet<E> tailSet(E fromElement) {
        synchronized (mutex) {
           return new SynchronizedSortedSet<>(ss.tailSet(fromElement),mutex);
        }
    }

    public E first() {
        synchronized (mutex) {return ss.first();}
    }
    public E last() {
        synchronized (mutex) {return ss.last();}
    }
}