Past is only used in tests. It was added speculatively in expectation that it would be useful for clients. Removing this means that all the fields of a Unit are present in the key, so the key can uniquely determine the Unit or even be parsed into one, and therefore multiple Unit instances can be equivalent if their fields match. If unit pasts are useful later, they can be tracked extrinsically the way timelines are tracked.
This keeps the rich features of a Season type without requiring constant string parsing (as much) or going through World to do lookups to get the objects. Since seasons now have value equality instead of reference equality, it's easier to get access to whem when needed. They're still, fundamentally, sugar over a tuple.
Tim Van Baak