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