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Almost Fill
Path: LifeAndDeath · Prev: Nakade · Next: KillingShapes
Keywords: Life & Death
A key concept for life and death is almost filling the eye space of a group, which means leaving one liberty inside the eyespace. Example
White's eyespace is almost filled with two stones. The circled point is the last liberty left inside the eyespace. It does not really matter whether the outside liberties are all filled or not. In this case they are, so White's group is in atari.
Here White has two outside liberties. The eyespace is still almost filled with two stones. What it means for life and deathIf you manage to almost fill the eyespace of an enemy group with a killing shape and there is no escape for that group, it is dead. KarlKnechtel: The reason this works is simple: Once the outside liberties are filled, if you (having almost filled) play next inside you capture; meanwhile, if opponent plays inside next at any time, your stones are captured - but - either the remaining space cannot yield life, or else it will not if you make the right play right after capture. ...Well. That's not the whole story, of course, because almost filling opponent's eyespace doesn't always make a killing shape. See killing shapes to know which shapes will kill a group. But what makes those shapes "killing shapes"? Simple. They have the following properties: - If opponent plays inside first with alternating play (and notice that if you've just been captured, it's your turn again), opponent has a sequence which makes it impossible to divide the space; - the shape can eventually be almost-filled with some other killing shape. Wait. That's a recursive definition. Quite right; we need a base case. Fortunately we have one: a single point is also a killing shape. Clearly it is not able to provide two eyes, and it is always almost-filled by definition. :) Okay. So what if I can't almost-fill with a killing shape? Well, there are a couple of possibilities. - If you can't do it because opponent can divide the space into two eyes, then opponent lives. Better not to try; you're just throwing stones away (except in some cases where you get a ko threat this way). - In some other (much less common) cases, you can almost almost fill (for lack of a better term) the eyespace, and make seki. If the outside liberties are filled before opponent can capture, you don't even need a killing shape for this. Authors
Path: LifeAndDeath · Prev: Nakade · Next: KillingShapes This is a copy of the living page "Almost Fill" at Sensei's Library. ![]() |