src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
@Override public K lastKey() {
        UnEntry<K,V> max = last();
        if (max == null) {
            throw new NoSuchElementException("this map is empty");
        }
        return max.getKey();
    }
@Override public K lastKey() {
        UnEntry<K,V> max = last();
        if (max == null) {
            throw new NoSuchElementException("this map is empty");
        }
        return max.getKey();
    }
Returns the last key in this map or throws a NoSuchElementException if the map is empty.