I was white. I screwed up the ending starting at 179, I know. I let him live, and I'm wondering how I could have killed him.

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My plan to become an SDK is here.

Moves 137 to 139 are the lowest level mistakes in the game.

At 184, the question can be reduced to: how many stones on the second line do you need to live, and when do you need to protect J17?

After that, 190 can go to F19, so there is only one gote eye on the edge (instead of a ko eye in sente). It would not matter, though, if you simply destroyed the second eye with F16 at move 198.

Black is still not alive at 209, and White just needs to live with B19. 210 is the final mistake.

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A good system naturally covers all corner cases without further effort.