src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
@Override public Option<UnmodMap.UnEntry<K,V>> entry(K key) {
        Node<K,V> t = tree;
        while (t != null) {
            int c = comp.compare(key, t.getKey());
            if (c == 0)
                return Option.some(t);
            else if (c < 0)
                t = t.left();
            else
                t = t.right();
        }
        return Option.none(); // t; // t is always null
    }
@Override public Option<UnmodMap.UnEntry<K,V>> entry(K key) {
        Node<K,V> t = tree;
        while (t != null) {
            int c = comp.compare(key, t.getKey());
            if (c == 0)
                return Option.some(t);
            else if (c < 0)
                t = t.left();
            else
                t = t.right();
        }
        return Option.none(); // t; // t is always null
    }
Returns an Option of the key/value pair matching the given key, or Option.none() if the key is
not found.