if (action == Actions.DO_NOTHING || action instanceof EmptySet || action == this) {
            return this;
        }
        if (action instanceof SingletonSet) {
            SingletonSet<T> singletonSet = Cast.uncheckedNonnullCast(action);
            return addOne(singletonSet.singleAction);
        }
        if (action instanceof SetWithFewActions) {
            SetWithFewActions<T> compositeSet = Cast.uncheckedNonnullCast(action);