Quick diversion maybe interesting to some rules-nerds:
If we are playing pure no-pass-go, without prisoner return, then that repeating sequence means that black is better off simply letting white capture the two stones. So in no-pass-go it’s actually not seki at all, and the normally bad T16 move is suddenly the correct local move to maximize points!
In that case the difference between seki and not seki wasn’t very dramatic. This position was more shocking to me:
In no-pass-go, the white group is dead! The reason is the same as above, black will play A3, and it’s better for white to allow black to capture the whole group (giving a large but finite number of local moves for black), than continually capturing (giving an infinite supply of local moves for black).Note that nothing changes if we add more black stones like this, the white group is still dead:
After we introduce prisoner return to no-pass-go, this weirdness disappears, since “sending two returning one” nets the opponent an extra prisoner, which they can then return as an extra move. So there’s no longer a way for black to get infinite moves when white resists capture, and all of these groups go back to being seki.
Knowing about these peculiarities of no-pass-go can actually be useful even if you’re playing regular go, if you happen to make a Molasses Ko in your game. They are exceedingly rare, but they have the potential to turn a regular go game into what is effectively a no-pass-go game, in which the above white groups would be dead, not seki.
(Check out this page for more details on these positions, plus some other fun stuff)
