class Box<E> {
    private E value;
    private boolean empty;

    public Box() {
	this.empty = true;
    }

    public Box(E value) {
	this.empty = false;
	this.value = value;
    }

    public E remove() {
	empty = true;
	return value;
    }

    public void insert(E value) {
	if (empty) {
	    empty = false;
	    this.value = value;
	}
    }
}