Seki inside the bulky five
A seki that will probably never see the light of day...
Jasonred : ... why... oh, fine, I'll indulge you. I guess that this shape can come about during poor playing when there are 3 very large kos in the game. Black plays in there, White ignores... no, wait, that's impossible too, since Black's first move kills the shape... in fact, why has Black just changed killing a group into a seki???
Bill: I doubt if this particular situation would arise on the board, but I seem to recall this shape occurring in a more complicated situation.
Charles I've found a near-miss in a pro game (Hashimoto Utaro-Iwamoto Kaoru 1936-05-31). Clearly enough the middle stone would be the final one played (otherwise Black could kill); but White's external liberties should also be filled, or White could prevent the seki by oshitsubushi. In the game I mentioned preventing the seki might have cost one point in teire.
Of course black can throw in at to make a (very bad for him) ko, but if black has any chance of gaining from that ko white would play at to make a different kind of seki.
Harry: What happens if White plays first at ? If Black connects at , then White can play, either at , or to the right of -- then Black dies. If Black does not connect at , then White immediately plays there, and Black will lose all their stones.
Bill: Harry, Black plays first, and so is already connected at . If White plays at instead of , we have the seki in the next diagram.