Just to note, the copy wasn't meant to deal with mutability but to note a changed interface. The items that are emitted have only a subset of the properties of

Just to note, the copy wasn't meant to deal with mutability but to note a changed interface. The items that are emitted have only a subset of the properties of those ingested. We could as easily have added some new properties to them as well. Mutability wasn't the primary concern here (although in Javascript it's always a genuine concern.)