EVENT
BOARDSIZE 19
SETUP W D17 D3 
SETUP B R16 Q3 
MARK 1@R16 3@Q3 4@D3 2@D17
COM
Problem 4. Black to play.  

White has played 4 in the lower left corner. What should Black
do now ? 
ENDCOM
B 1 P16
MARK B@O17 A@P17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black should take this opportunity to make a corner enclosure
with 1.

Instead of 1, making a corner enclosure with A or B would also
be a good move.
ENDCOM
W 2 R5
COM
Allowing Black to make a second corner enclosure in the lower
right corner would give him a large territorial advantage, so
White must make an approach move with 2.
ENDCOM
  VAR
  W 2 Q5
  COM
  Variation.

  If White makes a high approach move with 2, ...
  ENDCOM
  B 3 Q8
  MARK B@R5 A@R7
  COM
  Black will play a two-space pincer with 3 or a pincer at A. Black
  3 at B is also possible.
  <=
  ENDCOM
  ENDVAR
B 3 R9
COM
However, Black can now take the initiative by playing a pincer
with 3. This move is also an extension from his corner enclosure
above.
<=
ENDCOM