def get_collection_in_tower(self, key):
        """
        Get items from this collection that are added in the current tower.
        """
        new = tf.get_collection(key)
        old = set(self.original.get(key, []))
        # persist the order in new
        return [x for x in new if x not in old]
Get items from this collection that are added in the current tower.