@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);
    }
@ 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}
{