persistable sclass SynchronizedSortedMap<K,V>
    extends SynchronizedMap<K,V>
    is SortedMap<K,V>
{
    SortedMap<K, V> innerMap() { ret (SortedMap) m; }

    SynchronizedSortedMap(SortedMap<K,V> m) {
        super(m);
    }
    SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
        super(m, mutex);
    }

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

    public SortedMap<K,V> subMap(K fromKey, K toKey) {
        synchronized (mutex) {
            return new SynchronizedSortedMap<>(
                innerMap().subMap(fromKey, toKey), mutex);
        }
    }
    public SortedMap<K,V> headMap(K toKey) {
        synchronized (mutex) {
            return new SynchronizedSortedMap<>(innerMap().headMap(toKey), mutex);
        }
    }
    public SortedMap<K,V> tailMap(K fromKey) {
        synchronized (mutex) {
           return new SynchronizedSortedMap<>(innerMap().tailMap(fromKey),mutex);
        }
    }

    public K firstKey() {
        synchronized (mutex) {return innerMap().firstKey();}
    }
    public K lastKey() {
        synchronized (mutex) {return innerMap().lastKey();}
    }
}