This tsumego is a bit more difficult than the previous one (Gengen Gokyo #2), but there is a nice instructive move I want to show you. Black to play, thus the first move seems to be obvious, because it takes an eye directly:
The next move, however, could be a bit hard to see - but only if you do not take your time to read. Let's explore some possibilities
This variation looks too good for Black. White is dead. But isn't there a better response for White? What about the following:
If White decides that the whole group is enough for him, it is easy to get two or three eyes (depends if he decides to defend at X too). So Black 1 is not the solution.
This is obviously bad. It is needed to consider the bad moves, too, before we decide which one to play, though. The next variation might look interesting:
As you can see, Black 1 in the variation above destroys one eye, because A and B are miai - that means that if White plays at A, Black takes B; and if White plays at B, Black takes A. It lets White play elsewhere instead, though, which allows him to make two eyes and even respond Black 3 at 4.
The next variation is the correct one. Please try to think about Black 3 (because we continue solving the problem, I mark the move as 3, not 1).
As you can see, Black 3 forces White to play not one, but two moves - A and B - to be able to capture him. Moreover, White cannot play at X, because of the capture at C. Now, White has two options:
The first option is defending the eye at 4. Black can then kill White as follows.
The other option is try to kill the intruding Black stones by playing A and B from the diagram we discussed earlier:
Black 5 eliminates the eye. White 6 then threatens the capture. What should Black do now? The following is the instructive move I talked about in the beginning of this post:
Black 7 adds another stone to the kill. But see the price of the stone! Once White captures at 8, Black can kill the three corner stones, because the 7th gave him two killing points! Black 9 will be the last move of the tsumego, as White is left with just one eye then: