class Elements implements List<Element>, Cloneable {

    public Elements parents() {
        HashSet<Element> combo = new LinkedHashSet<Element>();
        for (Element e: contents) {
            combo.addAll(e.parents());
        }
        return new Elements(combo);
    }

    public  Elements();
    public  Elements(int initialCapacity);
    public  Elements(Collection<Element> elements);
    public  Elements(List<Element> elements);
    public  Elements(Element... elements);

}