EVENT
BOARDSIZE 19
SETUP W D17 F17 H17 D12 D7 D4 Q5 
SETUP B H15 F15 D15 D9 Q3 Q8 Q16 
MARK A@D9 B@D7
COM
Problem 70.  Black to play.  

The exchange of Black A for White B has taken place on the left.
How should Black continue ? 
ENDCOM
B 1 C17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black should attach with 1, ...
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  VAR
  B 1 E11
  MARK D@E17 C@D16 B@C18 A@C17
  COM
  Failure.

  If Black attacks at 1 without reinforcing as in the Correct Answer
  (A and C), ...
  ENDCOM
  W 2 C15
  COM
  ... White can attach and cut with 2 ...
  ENDCOM
  B 3 C14
  W 4 D14
  COM
  ... and 4, then ...
  ENDCOM
  B 5 C16
    VAR
    B 5 E14
    MARK \t@D14
    COM
    Variation.

    When White cuts with the marked stone, Black can't get a good
    result with the atari of 5 here.
    ENDCOM
    W 6 D13
    B 7 C16
    W 8 B15
    B 9 D16
    W 10 D11
    B 11 E10
    W 12 B16
    B 13 C17
    MARK B@K16 A@D18
    COM
    After the sequence to Black 13, White's stones on the left are
alive and she can secure her stones at the top by playing A or
    B.
    <=
    ENDCOM
    ENDVAR
  W 6 E15
  B 7 B15
  W 8 E14
  COM
  ... link up with the sequence to 8.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 F12
  COM
  Failure.

  Simply capping with 1 and ...
  ENDCOM
  W 2 E11
  B 3 G10
  COM
  ... playing the knight's move of 3 just enables White to get
  the initiative on the left side ...
  ENDCOM
    VAR
    B 3 F9
    MARK 2@E11 1@F12
    COM
    After exchanging 1 for White 2, Black could jump to 3.
    ENDCOM
    W 4 F11
    B 5 G11
    W 6 G12
    B 7 G10
    W 8 F13
    B 9 H12
    W 10 G13
    B 11 H13
    COM
    Black confines White to the left side with the sequence to 11.
    This is a reasonable result, but not as good as the one in the
    Correct Answer.
    <=
    ENDCOM
    ENDVAR
  W 4 E9
  MARK 2@E11
  COM
  ... with 2 and 4. The black stone on the left side still has
  some aji, but Black is now at a serious disadvantage there.
  ENDCOM
  ENDVAR
W 2 C18
B 3 D16
COM
... then reinforce his position by forcing with 3.
ENDCOM
W 4 E17
COM
White must answer with 4.
ENDCOM
B 5 E11
COM
Black can now use his thick position above to attack the white
stone with 5.
ENDCOM