A two-stone chain on the first line with both ends sealed by your stones: a convoy in a tunnel whose only exit is the second line above. Laddering a group is the same chase as laddering a lone stone, only better — the chain buys one liberty per extension while your two flanking stones keep the walls solid. Atari from the second line, over either stone, and the whole group must crawl toward the corner. Black to play: atari the white group to start the ladder.