So here we are in WWDC 2019 video 230, and starting at about minute 14 it is claimed that NSFetchedResultsController now vends an NSDiffableDataSourceSnapshot, so we can just apply it directly to a diffable data source (UITableViewDiffableDataSource).

But this is not quite what they say, or what we get. What we get, in the delegate method `controller(_:didChangeContentWith:)`, is an NSDiffableDataSourceReference. How do we get from this to an actual snapshot, and what should my diffable data source generic types be?