Herman: Correct me if I'm wrong, but isn't this a better defence for white? Now black's invasion has become pointless, and white gets 7 points, winning by 2. (Black must play at least one of the a marked points to prevent another point for white there)
You're right! Even more reason for B not to play this line. SiouxDenim
W cannot defend both attacks, so uses half a move to defend the cut and half a move to make an attack on the bottom B stones.
Herman: Shouldn't white's second half move be at a? That way, white can live and will not lose any stones...
Herman: Whoops, missed that :)
So how about this then? White will either make three eyes on the left, or get both moves a and get an eye there...
It's not easy this problem making malarky! I added the 3 White stones on the right to provide some threats, they weren't supposed to be an option for an eye :( I can't find an easy fix for the problem either. SiouxDenim
This is about the simplest problem I can think of. B to kill some W stones.
Matti: Black cannot kill.
SiouxDenim: Let's play it out and see.
Now Black has a parity problem. The group on the right needs 3 stones playing within it, with a single stone played on the first move. Black has no sensible places to play the other half of the move. When I first analysed this, I assumed that Black could pass for half of a move. The rules as currently written don't make this clear.
Alternatively, White can try to save both groups directly, by placing one stone on each side. Again, the aim is to get 3 eyes so a group can live.
There are obviously only 2 eye spaces now on the left, so W makes the group on the right obviously alive and asks B to prove the group on the left is not seki
I can see good arguments either way for allowing and disallowing half-pass moves; it'll only matter in games with very little territory. I propose that a pass is for a whole move.