erikpan: Are there any favoured responses to the somewhat strange 'kosumi' approach shown here? The only moves that spring to mind for me are a block at a (seems bad for shape) or a pincer somewhere around the points marked b.
To answer your question, replying at a to the White approach at is a basic shape move called a stretch. This stretch accomplishes several goals:
- increases Black's liberties while decreasing White's liberties
- begins to create a base for the Black stone in the corner
- denies White access to the corner...the 3-3 point is often critical to controlling the corner
After the stretch, the single White stone is significantly weaker.
Celebrir: It may be useful to see what happens if Black pincers.
Willemien: How about c (see more below)
This looks nearly like a 3-3 Point Invasion, which normally is good for Black. But what happens if White doesn't play at 3-3 ?
Now Black has two separated groups and White got the corner, which is a bad result for Black. Therefore the marked move was a bad one. Andy: White didn't "get" the corner since this shape is not alive in the corner yet.
willemien How about blocking at the other side and building a moyo along the upper side?
kb: In a game, I would block at the 3-3 95% of the time. Black then has 5 liberties to White's three, the whole corner, and the whole top side. The diagram below is the best White can do afterwards.
kb: If White pushes up Black practically wins the game. Look how much Black gains!