code: @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2) {
      return construct(Ordering.natural(), 2, e1, e2);
    }
docs: Returns an immutable sorted set containing the given elements sorted by their natural ordering.
  When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
  one specified is included.
  
  @throws NullPointerException if any element is null