I’ve just finished this nice game https://online-go.com/game/21473106
I have a question:
At move 31, my opponent killed A1 and A2. At move 32, I couldn’t play A2, showed as illegal move. Is this ko? What am I missing?
To prevent any misunderstandings, this is not a dispute of the outcome, I’m only asking to know next time it happens.
Chinese super ko rules prevent repeating board positions. This doesnt happen if you use japanese rules, those would allow the capture at A2.
Why is it a repeated position? Two stones gone in 31, one back in 32. What am I missing?
I honestly don’t get it. I’m not yet very comfortable with rulesets.
Superko sounds like a supervillain. 
You’re trying to repeat the board state that existed after move 29.
New question: how would someone keep track of this in a live game?!
It kind-of barely matters outside of a tournament context, where you’re more likely to have players who are either memorizing their games or keeping a kifu anyway.
In casual play if you miss this you’re not really hurting anything. I usually play Japanese rules anyway, I can’t say I’ve ever worried about “if only a super-ko could have prevented that.”
It requires a degree of memorization if doing things manually, but by playing on a computer, things are handled automatically.
Long cycles rarely exceed more than 6 moves (triple ko). The common shorter cases are restricted to a set of patterns that one could recognize to realize that the position is repeating.
Trust me to happen upon that too in the near future. 
Chinese rules (area scoring) dont care about the number of captures, so playing that exact sequence does work as annoying “2-steap ko threat” which doesnt change the score, but techically allows white to play it over and over again wihtout ever finish the game.
Japanes rules use territory scoring which takes the number of captures into account, so under japanese rules white would lose 1 point with each cycle, thus preventing white dragging game forever by just repeating it (since after enough repeats b could just ignore the whole thing and still call it a local win)
I confess I don’t pay attention to what rules in playing, shame on me I know. 
My question was more about when it does matter, what if it happened during a tournament and everyone missed it (it can happen, people make mistakes), do they dispute the game? Is there gossip on this have ever happened? 19x19 must be much harder to keep track anyway!
I won’t pretend I really, really get it yet, but I understand better now.
And if this happens in a game played with real board, its the same as with chess. In chess, you can declare a draw if board repeats 3 times, but this is totally up to players to do so.
It does not automatically end the game as a draw unless someone declares it. At tournament play this might mean that players have to start a new game, but that really depends on tournament rules.
Edit: i want to add that EGF tournaments use japanese rules, so i have never been in any tournament that uses chinese rules and i have absolutely no first-hand knowledge what would happen if super-ko rule comes into effect.
But you can at see how this sequence of “w a1, b a3, w a2, b pass” can be repeated over and over again without super-ko rule? (w can play a1 again after b pass)
See how black captures 2 stones where white only captures 1, so under japanese rules this sequence would be +1 point for black. So under jap rules, with enough cycles black could pass instead a3 and allow white to connect (thou b have to play c1 because stones are in atari)
But since chinese area scoring ignores all captures, and only counts the living stones and territories, this whole sequence is ±0 points for both players, because the board does not actually change at all.
Now imagine game being really close and black having only 0.5 point lead after all ‘proper’ moves having been played. If white would decide to start playing this cycle, and if black would not take a3, but instead play c1 and allow white to connect a3, this would mean white gaining 1 point by turning dame into living white stone, while b does not gain anything by turning 1 point of territory into living stone. White would win, so black would always have to take at a3 after w a1 while w could just play the cycle again and again.
For this reason chinese rules have the super-ko rule, it is to prevent that specific situation.
Yes, thank you. I won’t say I won’t make the same mistake again, but I do understand now.