EVENT
BOARDSIZE 19
SETUP W D16 D4 O17 P17 Q17 R17 
SETUP B Q4 Q10 Q16 R16 P16 L16 
MARK \t@O17
COM
Problem 26. Black to play.  

White has just played the marked stone. Taking in account his
sanren-sei formation on the right side, where should Black play
next ? 
ENDCOM
B 1 O16
COM
There is no other move for Black except to block at 1. Confining
White to the top is absolutely essential in the sanren-sei strategy.
ENDCOM
  VAR
  B 1 K4
  COM
  Failure.

  Black 1 is certainly a big point and Black would like to play
  there first, but ...
  ENDCOM
  W 2 O16
  MARK \t@L16
  COM
  ... White will turn at 2 and the marked stone at the top will
  be isolated from its allies on the right. Moreover, the three
  black stones to the right of 2 are now heavy because they are
  short of liberties.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 N15
  MARK @@O16
  COM
  In the right direction.

  Black 1 here is in the right direction, but it is not as tight
  as a move at @ in the Correct Answer.
  <=
  ENDCOM
  ENDVAR
W 2 N16
COM
White resists with 2 ...
ENDCOM
B 3 N15
W 4 M15
COM
... and 4 to get some aji in the center.
ENDCOM
B 5 M16
W 6 N17
B 7 N14
COM
With Black 7, the joseki comes to a pause.
ENDCOM
W 8 O3
COM
White 8 is now the most natural move.
<=
ENDCOM