SortedMapDifference<K, V> difference(
          SortedMap<K, ? extends V> left, Map<? extends K, ? extends V> right) {
    checkNotNull(left);
    checkNotNull(right);
    Comparator<? super K> comparator = orNaturalOrder(left.comparator());
    SortedMap<K, V> onlyOnLeft = newTreeMap(comparator);
    SortedMap<K, V> onlyOnRight = newTreeMap(comparator);
    onlyOnRight.putAll(right); // will whittle it down