I was looking at this pro game:
White wins by half a point. The last few moves are as follows:
- Click Here To Show Diagram Code
[go]$$Wcm1 Moves 226 to 235
$$ ---------------------------------------
$$ | . . . . . . . . . . . . O X X X X X . |
$$ | . . . . . . . O O . . O O O O O O X . |
$$ | O . O . O O O X O X O O X X O O X X . |
$$ | X O . O X X X X X O O X . X O X X . X |
$$ | X O O X . 8 7 O X X X X X X X O X X O |
$$ | X O X X . . X O O O X O O . X O X X O |
$$ | X X X . . . X X O . X O O X X O X O O |
$$ | X O O X . . X O O O O 9 . O O O O . O |
$$ | O . O X . X X X O O X O . . O O O . O |
$$ | . . O X . . X O O X X X O . . , . O . |
$$ | . . O X . X X X O 5 6 O 3 O O . . . . |
$$ | . . O O X . O O X X X O 2 X O . . O . |
$$ | . O X X X X O X . . . X . X O O . . O |
$$ | . O X X X O X . X . . . X O X X O O 1 |
$$ | . O O X O O X X . . . . . 4 X O X X O |
$$ | . . O O O O O X . , . . . . . , X . X |
$$ | . . X X O X X . . . . . . X . X . . . |
$$ | . . . O . O X X X . . . . . . . . . . |
$$ | . . . . O O O O X . . . . . . . . . . |
$$ ---------------------------------------[/go]
My question is, why didn't black play

at

? Why does black have to protect?
- Click Here To Show Diagram Code
[go]$$Wcm1 Block and win the game?
$$ ---------------------------------------
$$ | . . . . . . . . . . . . O X X X X X . |
$$ | . . . . . . . O O . . O O O O O O X . |
$$ | O . O . O O O X O X O O X X O O X X . |
$$ | X O . O X X X X X O O X . X O X X . X |
$$ | X O O X . . 2 O X X X X X X X O X X O |
$$ | X O X X . . X O O O X O O . X O X X O |
$$ | X X X . . . X X O . X O O X X O X O O |
$$ | X O O X . . X O O O O . . O O O O . O |
$$ | O . O X . X X X O O X O . . O O O . O |
$$ | . . O X . . X O O X X X O . . , . O . |
$$ | . . O X . X X X O . . O . O O . . . . |
$$ | . . O O X . O O X X X O . X O . . O . |
$$ | . O X X X X O X . . . X . X O O . . O |
$$ | . O X X X O X . X . . . X O X X O O 1 |
$$ | . O O X O O X X . . . . . . X O X X O |
$$ | . . O O O O O X . , . . . . . , X . X |
$$ | . . X X O X X . . . . . . X . X . . . |
$$ | . . . O . O X X X . . . . . . . . . . |
$$ | . . . . O O O O X . . . . . . . . . . |
$$ ---------------------------------------[/go]
I can only assume there's some problem around N7 that keeps black from getting the last point and winning the game, but I can't find anything:
- Click Here To Show Diagram Code
[go]$$Wcm1
at 
$$ ---------------------------------------
$$ | . . . . . . . . . . . . O X X X X X . |
$$ | . . . . . . . O O . . O O O O O O X . |
$$ | O . O . O O O X O X O O X X O O X X . |
$$ | X O . O X X X X X O O X . X O X X . X |
$$ | X O O X . . 2 O X X X X X X X O X X O |
$$ | X O X X . . X O O O X O O . X O X X O |
$$ | X X X . . . X X O . X O O X X O X O O |
$$ | X O O X . . X O O O O . . O O O O . O |
$$ | O . O X . X X X O O X O . . O O O . O |
$$ | . . O X . . X O O X X X O . . , . O . |
$$ | . . O X . X X X O . . O 7 O O . . . . |
$$ | . . O O X . O O X X X O 6 X O . . O . |
$$ | . O X X X X O X . . 9 X 5 X O O . . O |
$$ | . O X X X O X . X . 0 . X O X X O O 1 |
$$ | . O O X O O X X . . . . . 3 X O X X O |
$$ | . . O O O O O X . , . . . . 4 , X . X |
$$ | . . X X O X X . . . . . . X . X . . . |
$$ | . . . O . O X X X . . . . . . . . . . |
$$ | . . . . O O O O X . . . . . . . . . . |
$$ ---------------------------------------[/go]
Is there some shortage of liberties or something that I'm not seeing?