src/main/java/org/organicdesign/fp/collections/PersistentTreeMap.java
@Override public Option<UnEntry<K,V>> head() {
        Node<K,V> t = tree;
        if (t != null) {
            while (t.left() != null) {
                t = t.left();
            }
        }
        return Option.some(t);
    }
@Override public Option<UnEntry<K,V>> head() {
        Node<K,V> t = tree;
        if (t != null) {
            while (t.left() != null) {
                t = t.left();
            }
        }
        return Option.some(t);
    }
{@inheritDoc}