code: @SuppressWarnings("unchecked")
    public static <E extends OrdinalValue<E>> ImmutableOrdinalSet<E> of() {
      // safe unchecked: will contain no elements
      return (ImmutableOrdinalSet<E>) EMPTY_SET;
    }
docs: Returns singleton empty immutable ordinal set
  @param <E> element type
  @return empty set