ListIterator::__construct($list){
    $list->addIterator();
}
ListIterator::__destruct(){
    $this->list->removeIterator();
}
List::remove($index){
    if($this->iteratorsCount > 0){ throw new ConcurrentModificationError(); }
    /* ... */
}

vs

ListIterator::__construct($list){
    $this->watch = function(){ $this->list = clone $this->list; };
    $this->list->addBeforeChange($this->watch);
}
ListIterator::__destruct(){
    $this->list->removeBeforeChange($this->watch);
}
List::remove($index){
    $this->notifyChanging();
    /* ... */
}