EVENT
BOARDSIZE 19
SETUP W C15 E16 F16 G16 H15 H14 J14 K14 Q4 R5 N3 P17 Q17 N17 
SETUP B D17 E17 F17 H17 H16 J15 K15 L15 L14 D4 F3 R10 R17 R16 Q16 P16 
MARK \t@H17 \t@H16 \t@J15 \t@K15 \t@L15 \t@L14 \t@N17 \t@P17 \t@Q17
COM
Problem 61. Black to play.  

Black's marked stones are thick, so the marked white ones are
in trouble. Where should Black start his attack ? 
ENDCOM
B 1 M18
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 on the second line is the vital point because it threatens
White's base. Moreover, because the black stones are thick, White
cannot easily escape into the center.
ENDCOM
  VAR
  B 1 L17
  COM
  Failure.

  Black 1 is slack.
  ENDCOM
  W 2 R18
  COM
  After exchanging 2 ...
  ENDCOM
  B 3 S18
  COM
  ... for 3, ...
  ENDCOM
  W 4 O16
  MARK B@O14 A@M18
  COM
  ... White plays 4. A and B are now miai.



   - if Black A, White escapes with B; if Black B, White makes
  two eyes with A.
  <=
  ENDCOM
  B 5 M18
  COM
  If Black 5 here, ...
  ENDCOM
    VAR
    B 5 O14
    COM
    If Black 5 here, ...
    ENDCOM
    W 6 M18
    COM
    ... White makes two eyes with 6.
    <=
    ENDCOM
    ENDVAR
  W 6 O14
  COM
  ... White escapes with 6.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 O16
  COM
  Failure.

  Black 1 is a vital point, but, ...
  ENDCOM
  W 2 R18
  COM
  ... after exchanging 2 ...
  ENDCOM
  B 3 S18
  COM
  ... for 3, ...
  ENDCOM
  W 4 M18
  COM
  ... White plays 4 and gets two eyes.
  ENDCOM
  B 5 K18
  COM
  That is, if Black 5 here, ...
  ENDCOM
    VAR
    B 5 Q18
    MARK \t@R18
    COM
    If Black capture the marked stone with 5, ...
    ENDCOM
    W 6 P18
    COM
    ... White ataris once, ...
    ENDCOM
    B 7 R19
    W 8 K18
    COM
    ... then jumps to 8.
    <=
    ENDCOM
    ENDVAR
  W 6 R19
  COM
  ... White defends at 6.
  ENDCOM
  ENDVAR
W 2 O16
COM
If White bulges out with 2, ...
ENDCOM
  VAR
  W 2 M17
  COM
  Variation.

  If White 2 here, ...
  ENDCOM
  B 3 L18
  COM
  ... Black will extend to 3.
  <=
  ENDCOM
  ENDVAR
B 3 O15
COM
... Black keeps the white stones confined with 3 ...
ENDCOM
W 4 N15
B 5 N14
COM
... and 5.
ENDCOM
W 6 R18
B 7 S18
W 8 O14
B 9 P15
W 10 N18
B 11 M15
W 12 L17
B 13 M17
W 14 M16
COM
The sequence continues to 14, but ...
ENDCOM
B 15 N16
COM
... Black starts a ko with 15. White has no ko threats, so Black
will capture all of White's stones.
<=
ENDCOM