Thanks, Harlequin. :-)

A pity that the pass for ko rule seems to be necessary. However, the once only rule prevents this kind of infinite play.

I think of it more as a kind of disturbing ko rule. ;-)

In human terms, there is a difference between superko rules and rules such as this once only rule and my end of game ko rule. Superko rules are more general and harder to deal with because of that. The once only rule only requires players to keep track of whole board positions where a player took a ko, while a superko rule requires players to keep track of every position. In fact, in virtually all cases the sequence of play which leads to the once only prohibition will consist of ko plays. It is much easier to apply than a superko rule.

Some rules define a ko as a repetition of a whole board position two moves earlier. (The Japanese rules do not.) With such rules I could simply prohibit taking the same ko twice.