for-in, Collection conformance, etc also expose an order, and I showed that your example of why prefix could be harmful applies equally to for-in. I keep asking about how you are separating harmful API from benign here, but I haven't worked it out yet. I would understand if the argument was that all such order-exposing API be quarantined on a Collection and Sequence conforming view, and I'm not unsympathetic to that view, but I'm struggling to understand why some seemingly random assortment need to be removed. And which assortment should it be? I've given an example of how prefix can be perfectly useful on a Set, for example.