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12.17 Ladders in Joseki

As an example of how to use autohelpers with the Joseki compiler, we consider an example where a Joseki is bad if a ladder fails. Assume we have the taisha and are considering connecting on the outside with the pattern

 
--------+
........|
........|
...XX...|
...OXO..|
...*O...|
....X...|
........|
........|

But this is bad unless we have a ladder in our favor. To check this we add a constraint which may look like

 
--------+
........|
........|
...XX...|
...OXO..|
...*OAC.|
....DB..|
........|
........|

;oplay_attack(*,A,B,C,D)

In order to accept the pattern we require that the constraint on the semicolon line evaluates to true. This particular constraint has the interpretation "Play with alternating colors, starting with `O', on the intersections `*', `A', `B', and `C'. Then check whether the stone at `D' can be captured." I.e. play to this position

 
--------+
........|
........|
...XX...|
...OXO..|
...OOXX.|
....XO..|
........|
........|

and call attack() to see whether the lower `X' stone can be captured. This is not limited to ladders, but in this particular case the reading will of course involve a ladder.

The constraint diagram above with letters is how it looks in the `.db' file. The joseki compiler knows how to create these from labels in the SGF node. `Cgoban' has an option to create one letter labels, but this ought to be a common feature for SGF editors.

Thus in order to implement this example in SGF, one would add labels to the four intersections and a comment:

 
;oplay_attack(*,A,B,C,D)

The appropriate constraint (autohelper macro) will then be added to the Joseki `.db' file.


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