InvasionProblem1/A

Small scale  

Black's attack at makes it inadvisable for White to try to save directly, but as an attack, it is too small-scale. Yamashita says White will just play . White's corner group will be unassailable, so if Black continues by capturing with a move like a, White will be happy to take sente - Black will have committed the sin of using thickness to make territory. If Black plays tenuki himself at this point, White can look forward to attaching at b later.