public SortedSet<E> create(Object... elements) {
      List<?> normalValues = (List<?>) asList(elements);
      List<E> extremeValues = new ArrayList<>();

      // nulls are usually out of bounds for a subset, so ban them altogether
      for (Object o : elements) {
        if (o == null) {
          throw new NullPointerException();
        }
      }

      // prepare extreme values to be filtered out of view