5. In their "dessert" section, they talk about a problem with sort when the items are shallow clones. It's an example of a broader problem: they put something i

5. In their "dessert" section, they talk about a problem with sort when the items are shallow clones. It's an example of a broader problem: they put something into an `ObservableVector` but then semantically mutate it via inner mutability (defeating the "observable"). You just can't do that. The sort infinite loop is the tip of the iceberg. Everything relying on the observable aspect is then wrong. The lesson isn't just "jumping on an optimization can lead to a bug"; it's also that abstractions have contracts.