Yeah that's the usual terminology in JS world. Persistence (the naming) comes out of algorithm theory, which IMO has pretty poor naming in general (dynamic prog

Yeah that's the usual terminology in JS world. Persistence (the naming) comes out of algorithm theory, which IMO has pretty poor naming in general (dynamic programming and cache-oblivious also come to mind as unfortunate names). Not entirely sure why that is. There's a few "levels" of persistence available, just as an aside. Immutability I think corresponds to the ~strongest level, but also means it can be hardest to achieve. One easier version allow only _querying_ old versions of the data structure for instance, no modifications except at the tip. Sometimes that's enough.