Coping with an invalid invasion

The diagram to the right is taken from the latter stage of an actual game. Black, having played GCS was apparently well ahead. White 1 was played more in hope than in expectation. Black has a weakness at A so how should he respond?

The diagram below shows what actually happened next ...

Continuation

Black 2 defended by a simple block. After white 3, black 4 blocked in a similar fashion. White 5 threatened to connect and so forced black 6. White 7 guaranteed an eye at B. Black 8 (atari) attempted to reduce the eye space. But instead of connecting white 9 aims at achieving a second eye at D. At the next move Black can capture by playing at C but this is a ko. Life via ko in what was Black's corner is a considerable gain for White. So where did Black go wrong? See next diagram.

The correct approach

The one-skip black 2 in the final diagram is the correct idea. It not only protects the weakness but also cramps White's potential eye space. After black 10, White must split the available space into two separate eyes, by playing at both E and at F. But if White takes one point Black can take the other and thus ensure that White gets only one eye and so dies!

The secret to finding moves such as black 2 is twofold. First, having previously adhered to GCS, the player should positively expect to have strong positions and therefore not assume that the opponent can validly invade. Second, the player must actively consider candidate stones that can effectively be connected but nevertheless undermine the invading group's eye space.