After , which makes an eye at the circled point, a and b are miai for life. Instead of
, c also lives, but allows white to seal off the right side: see below.
Because there are two possible solutions, this problem may be called unsound if you use a specific definition for sound/unsound problems.
It seems as if creates a false eye, but the capture of three stones on the first line with
results in a real eye.
White's best endgame followup is to capture a Black stone with . Then
or a makes a second eye. However, if Black doesn't respond, White a kills the group. After
, black has just over 3 1/2 points in the corner (later, either side may play b in gote).
at the other 1-2 point is wrong. With
, Black is dead. Even if Black is allowed two moves in a row, he can't live.
When Black plays here, White kills with
. (
at
is an inferior way to kill, because after Black
and White a, Black gets some concrete threats at c.)
To this way of living, White can seal the right side in sente. is necessary to avoid ko: see bent four in the corner. This is the reason why the main line solution is usually preferred.
Early in the game, if there are other big moves remaining on the board, KataGo actually prefers here rather than the main line. The reason is that
in the diagram above won't be sente if black is in a good position to fight the ko. Instead, white has this
and
as forcing moves. Black has only 3 points of corner territory, compared to nearly 4 points in the main line; however, black can push out at b in sente. So in some contexts, this choice of
is actually more profitable than the main line.