EVENT
BOARDSIZE 19
SETUP W C18 D17 E18 G16 C5 F5 Q4 
SETUP B C17 C16 D15 C9 D3 G3 R16 
COM
Problem 166. Black to play.  

Black should be happy about his result on the left side. Can
he now switch to the right side, or is there anything else still
remaining to be done on the left ? 
ENDCOM
B 1 D12
MARK \t@G3 \t@F5
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Defending with Black 1 is the best move. This makes the exchange
of the marked stones bad for White. White's two stones on the
lower left are now caught between two strong black positions.
<=
ENDCOM
  VAR
  B 1 P17
  COM
  Failure.

  Making a corner enclosure with Black 1 in not good. Black's position
  on the left is too thin.
  ENDCOM
  W 2 C12
  COM
  White will invade with 2 and ...
  ENDCOM
    VAR
    W 2 D12
    MARK A@B12
    COM
    Too high.

    White 2 on the fourth line is too high. If Black's stones get
    into trouble, he can always link up at A.
    <=
    ENDCOM
    ENDVAR
  B 3 F15
    VAR
    B 3 E12
    MARK 2@C12
    COM
    The cap.

    Black might try to counter White's invasion at 2 by capping
    with 3, but ...
    ENDCOM
    W 4 D11
    B 5 E9
    W 6 E11
    COM
    ... White simply moves out into the center.
    <=
    ENDCOM
    ENDVAR
  W 4 E12
  B 5 E9
  W 6 G12
  COM
  ... Black is left with two weak groups to defend after White
  6.
  <=
  ENDCOM
  ENDVAR