Problem diagram.
Black's response to White's marked play?

Source:
American Go Journal, Vol. 19, No. 2, 1985, p. 18
Sketching Out the Game , Kobayashi Chizu

Solution follows    (or click here to download SGF)

Correct.
The rule is to block on the longest side. The sequence through Black 9 is natural. White has territory and Black has a wall. Black's last move at 9 could also have attacked White at A.

Horrible.
Black 1 is a huge misdirection of play. Through the natural sequence to 9, White has greatly reduced the potential of the Black's stones at the bottom, while Black's plays have exerted influence in a direction away from his other stones.

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