@SuppressWarnings("unchecked")
    private int compare(E one, E other) {
        if (comparator == null) {
            return ((Comparable<E>) one).compareTo(other);
        } else {
            return comparator.compare(one, other);
        }
    }
@see java.util.AbstractCollection#add(java.lang.Object)
src/nu/validator/collections/TailBiasedSortedSet.java