code: @Override
  	public boolean addAll(Collection<? extends T> c) {
  		if (elements instanceof List<?>) {
  			//TODO: copiare codice di union
  			Collection<T> res = union(c).elements;
  			boolean r = !res.equals(elements);
  			elements = res;
  			return r;
  		}
  		return elements.addAll(c);
  	}
docs: {@inheritDoc}