In the 6th game the position after black 17 is exactly
the same as the game
B: Cho Chikun vs. W: Cho Sonjin,
1999, B+7.5 (Honinbo title match, 3rd game) with one
exception: the r17/l17 exchange.
It is interesting to see how especially white tries
to improve on that game. White 18, the ikken-tobi at
k6 is an attempt to get sente locally (white g4 is a
threat) to be able to take the teritorially important
move of d14. A possible second improvement is white's
low invasion of the lower-right corner with 20. The
resulting black thickness has weaknesses so black's
reinforcement with o7 is natural and white has time to
reinforce the group around k6. However, black happy to
take the initiative with the double-kakari at f17.