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The second step--part 5

This series for beginners started about two months ago. We have learned not only how to capture stones, but also how to prevent them from being captured by making two eyes. However, one problem remains. Some readers may have noticed something strange in one of last week's solutions.

Diagram 1: This is Solution 3D from last week. I said all of Black's marked stones were dead.

Diagram 2: The reason was that White could capture the marked stone with 1.

Diagram 3: This would be the new situation. White has just played the marked stone. Notice that this reduces the liberties of Black's remaining stones to just one.

Diagram 4: But, you argue, can't Black recapture the marked stone by playing at 2? This would bring us back to Diagram 1, and everything would start all over again.

Ko rule

In fact, the situation described here occurs frequently and is called ko. To prevent the capturing back and forth of ko, there is a rule: "No full board situation can exist twice." What does this mean?

Diagram 5: The implications of the ko rule are that after white 1, Black can't capture right away, because this would repeat a board situation. However, he can play somewhere else, for example at 2. It is up to White to answer black 2, but he can also capture with white 3.

Diagram 6: In the previous diagram, White didn't specifically need to answer. However, in this situation, after White captures the marked stone with 1, black 2 threatens to capture the two marked white stones (while reducing the whole white group to only one eye). In this case, it is a good idea to answer with 3.

Diagram 7: Compared to Diagram 3, the full-board position has changed--for example, the exchange 2 for 3 in Diagram 6--which makes it possible for Black to recapture the marked stone with 4. Now White can't recapture. White 5 may not seem like a compelling threat, but the problem is Black can't ever win the ko.

Diagram 8: For example, if he connects his marked stone by playing at 1, he puts all of his stones in immediate danger as there is only one liberty left at A. In other words, White could immediately play at A and capture the lot. This means White can play the ko at any time he pleases. That is the real reason for the demise of the Black stones.

Diagram 9: This is the basic ko shape. White can capture one stone by playing at 1. On the other hand, Black can't recapture immediately. He first has to play another move. White could then answer that move or he may choose to connect white 1 (by playing where the captured black stone was).

Problems

Problem 1: How can White start a ko?

Problem 2: Is there any way for Black to save his marked stones?

Problem 3: The moves to 5 are a standard method for invading this corner. How should White continue after black 6? See Reference 1.

Reference 1: After black 6, if White connects with 7, Black can kill the whole white group by playing at 8 and 10. A cut at 11 is not as dangerous as it looks and after black 12, White has run out of steam. His group is dead.

The English-speaking go community congregates every Sunday starting at 11 a.m. at Ben's Cafe in Takadanobaba, Tokyo. (Tel: 03-3202-2445 or visit www.benscafe.com).

Rob van Zeijst is a four-time European go champion and European representative at the Fujitsu World Championship.