src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
public UnEntry<K,V> last() {
        Node<K,V> t = tree;
        if (t != null) {
            while (t.right() != null)
                t = t.right();
        }
        return t;
    }
public UnEntry<K,V> last() {
        Node<K,V> t = tree;
        if (t != null) {
            while (t.right() != null)
                t = t.right();
        }
        return t;
    }
Returns the last key/value pair in this map, or null if the map is empty.