Michael Reiss wrote:
> Are there any tsume problems where the defender *must* pass but the pass
> move is not the defenders last move before the search terminates?
If a position is a seki in its terminal state, the only good move is a pass. So the defender *must* pass. Then the attacker tries all possible moves (all unreasonable but they have to be tried in order to complete the search). If some of these moves threaten to capture or get a ko, the defender has to answer them.
6 . . . . . .
5 X X X . . .
4 O O X X . .
3 . O O X X .
2 O X O O X .
1 . X . O X .
a b c d e f
For example: O passes,
X c1 threatens to capture
or
X a1 threatens to create a ko.
Both force O to answer.
I am sure some less obvious seki positions exist, where the attacker can play sente moves without putting himself in atari.
> Are there any tsume problems where the defender *must* pass *more than
> once* (not necessarily consecutively) in order to prove it's alive or
> seki?
There _may_ be some sort of unstable seki. A move played by the attacker would force the defender to answer. This move would then force a move by the attacker to keep the position a seki. Then the defender passes again. If the defender's position still has some weakness, the attacker can force a defensive move again. This is difficult to construct. Maybe the rules experts could help on the go rules mailing list?
Jean-Pierre Vesinet