*
 * @param <T> the type of the subject of the action
 */
public abstract class ImmutableActionSet<T> implements Action<T>, InternalListener {
    private static final int FEW_VALUES = 5;
    private static final ImmutableActionSet<Object> EMPTY = new EmptySet<Object>();

    /**
     * Creates an empty action set.
     */
    public static <T> ImmutableActionSet<T> empty() {