EVENT
BOARDSIZE 19
SETUP W D17 D4 C7 C16 C15 F17 
SETUP B Q16 Q4 D15 C14 D13 C9 
MARK 1@D15 4@C16 2@C15 3@C14 5@D13 6@F17 7@C9 8@C7
COM
Problem 21. Black to play.  

If Black plays the joseki to 7, how should he respond to White
8 ? 
ENDCOM
B 1 E9
MARK A@C12
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White is threatening to invade at A, so Black should defend his
position on the left side by jumping to 1.
ENDCOM
  VAR
  B 1 Q10
  COM
  Failure.

  It is tempting to make a sanren-sei on the right side with 1,
  but ...
  ENDCOM
  W 2 C12
  COM
  ... in that case White would invade at 2 and ...
  ENDCOM
  B 3 B13
  W 4 D10
  B 5 C10
  W 6 C11
  B 7 B11
  W 8 B12
  B 9 A12
  W 10 B10
  B 11 A11
  W 12 D9
  B 13 B9
  W 14 C8
  MARK \t@F17
  COM
  ... make thickness with the sequence to 14. This thickness and
  White's marked stone now serve to diminish the power of the sanren-sei
  formation on the right side.
  <=
  ENDCOM
  ENDVAR
W 2 R10
MARK B@R8 B@R12
COM
White would then invade at 2. From this position, Black would
play one of the points marked B and a joseki which involves the
whole right side would be put in motion. (See Get Strong at Joseki
3, Joseki 9.)
<=
ENDCOM