public boolean addAll(BstSet<? extends E> c) {
        for (E element : c) {
            add(element);
        }
        return true;
    }

    public E higher(E element) {
        E l = root.element;
        BstNode<E> max = getMin(root);
        if(element == max.element)
            return null;
        for (E e : this) {
            if(e.compareTo(element) >0){
                l = e;
                break;
            }
        }
        return l;
    }

    public E pollLast() {
        E lowest = root.element;
        BstNode<E> max = getMax(root);
        remove(max.element);
        return max.element;
    }