element* insert_sorted(element *first, element *new_elem) {
    if (first !=NULL && first -> isbn < new_elem -> isbn){
        element *current = first;
        while (current -> next != NULL && current -> next -> isbn < new_elem -> isbn){
            current = current -> next;
            new_elem -> next = first;
            first = new_elem;
        }
    new_elem -> next = current -> next;
    current -> next = new_elem;
    }
    } else {
        new_elem -> next = first;
        first = new_elem;
    }

    return (first);
    /* HIER implementieren. */
}