EVENT
BOARDSIZE 19
SETUP W Q3 E3 C18 D18 C17 C16 D15 C15 E17 F17 F18 
SETUP B R16 C4 D5 C11 D17 D16 E16 E15 E18 G16 G14 D14 
COM
Problem 101. White to play.  

The joseki in the upper left corner has come to a pause. White
has taken profit, but Black has thickness in the center. What
should White do next ? 
ENDCOM
W 1 G4
MARK A@F4
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Although it is important to quickly make an approach move or
a corner enclosure in the fuseki, White 1 is an urgent move that
must be played before any other. It is played to counter Black's
thickness at the top and to prevent him from pressing at A.
<=
ENDCOM
  VAR
  W 1 R5
  MARK A@P16
  COM
  Failure.

  If White makes a corner enclosure at 1 or an approach move at
  A, ...
  ENDCOM
  B 2 F4
  COM
  ... Black will immediately play 2 ...
  ENDCOM
  W 3 G3
  B 4 H4
  COM
  ... and 4, and an enormous moyo comes into being on the left.
  The moves to Black 4 are a joseki. (See Problem 49 in Get Strong
  at Joseki 1.)
  ENDCOM
  ENDVAR