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 makes the flow of data easier to read and is slightly shorter in some cases. This pattern will be better for replacing more reference members with keys.
There are probably still reference equality issues here since Unit still has Season and Power objects. The test case builder also still works on reference equality in some places so the second part of adjudication is broken.
Tim Van Baak