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I believe the implementation of "pure winning" is somewhat flawed. Indeed you want to avoid getting a "winning number" by having beaten a strong player when he was still weak. But players' strength not only increases, it can decline ... So even if Guo defeated Rui before she beat Otake, it is not 100% sure this is less pure than when she would have beaten Rui afterwards.
True, but generally it is safe to say that a player's strength will decline much less than it has increased. I think it is rare for a player to get more than 1-2 stones weaker from a level they have reached. So although using the time of victory is no guarantee of absolute strength, it at least comes close, and there really isn't any other way, I think. Professional dan grades are for life, as are many amateur dan grades.