src/main/java/org/organicdesign/fp/collections/PersistentTreeSet.java
@Override
    public UnmodSortedIterator<E> iterator() {
        return new UnmodSortedIterator<E>() {
            UnmodSortedIterator<? extends UnmodMap.UnEntry<E,?>> iter = impl.iterator();
            @Override public boolean hasNext() { return iter.hasNext(); }
            @Override public E next() {
                UnmodMap.UnEntry<E,?> e = iter.next();
                return e == null ? null : e.getKey();
            }
        };
    }
@Override
    public UnmodSortedIterator<E> iterator() {
        return new UnmodSortedIterator<E>() {
            UnmodSortedIterator<? extends UnmodMap.UnEntry<E,?>> iter = impl.iterator();
            @Override public boolean hasNext() { return iter.hasNext(); }
            @Override public E next() {
                UnmodMap.UnEntry<E,?> e = iter.next();
                return e == null ? null : e.getKey();
            }
        };
    }
{@inheritDoc}