class ListHead<K,V> {
    private List2<K,V> list = null;
    private RW rw = new RW();
    public V lookup (K key) throws InterruptedException {
	rw.startRead();
	V result= (list==null) ? null : list.lookup(key);
	rw.endRead();
	return result;
    }	
    public void update (K key, V value) throws 
        		InterruptedException {
	rw.startWrite();
	if (list == null)
            list = new List2<K,V>(key,value,null);
	else list.update(key,value);
	rw.endWrite();
}   }