: ( 415)
It is correct for White to turn towards the left side.
:
Black closes the upper left corner, attempting to return to the path of the "usual" semeai between right and left.
: (A 418)
White captures Black's stones on the left side. Black has secured his top left corner already, so White cannot win with following the main path of our solution.
: However, she should have put more effort in her choice here, with a potential ko-fight in sight.
:
Black's previous move at (= ) not only took White all of her potential ko-threats in this area, but also reduced her upper partial group to only five liberties, so generating even more ko-threats for Black than "usual". Therefore, White cannot consider the ko-variations any more, but is forced to live with her group on the upper edge.
:
Black's large group in the top right is alive now, so White starts a close endgame.
(L) 
415
:
: (A 
418
:
:
) not only took White all of her potential ko-threats in this area, but also reduced her upper partial group to only five liberties, so generating even more ko-threats for Black than "usual". Therefore, White cannot consider the ko-variations any more, but is forced to live with her group on the upper edge.
:
