Difference between revisions of "Foiling"

Revision as of 19:14, 28 December 2020

To foil a ladder escape means to make a move which prevents an outpost from being used as a ladder escape, and also intrudes on the outpost's connection to the edge.

Example

Consider the following position:

Red has just played f6. In his next move he can either start a ladder at c7, using f6 as a ladder escape, or he can play g4, making an unbreakable connection from top to bottom. Thus f6 threatens two different connections.

However it does not secure Red a connection, because there is one vulnerable cell, namely e7:

If Blue plays here, he prevents the use of f6 as a ladder escape, and he also intrudes on its edge template to the bottom. In fact in this position Blue wins.

So to foil a ladder escape you make move on the row below the outpost, in the direction of where the ladder will be coming from. Are there other foils?

Foiling does not always work

Consider the following position, which is almost equal the one in the first diagram:

If Blue tries to foil f6 now, Red responds at f7:

Observe that the ladder still works, and so does the connection via g4. Since Blue only can stop one of these two, Red wins.

When do foils work?

In general, it is difficult to figure out when a ladder escape can be foiled. There are some simple rules that apply in some cases.

• A forking ladder escape on the second row is unfoilable.

• A forking ladder escape on the third row is unfoilable if the cell marked "*" is empty, and is not required for the "connection up":

However, if the cell marked "*" is occupied by Blue, the ladder escape fork can often be foiled; in that case, playing at "+" is the only way of foiling it. Also, if the cell marked "*" is empty, but is required for Red's threatened upward connection, the fork may be foilable by playing at "+", as in the following example:

Because the cell marked "*" is required for Red's threatened connection to 10, the ladder escape fork is foilable by playing at "+" (but not by playing at "*").

• A forking ladder escape on the fourth row is more complicated. If the approaching ladder is a 2nd row ladder, the fork is unfoilable if the cells marked "*" both are empty (and not required for the "connection up"). Otherwise, it may be foilable, and in that case, playing in one of the cells marked "+" is the only way to foil it.

If the approaching ladder is a 3rd row ladder, the fork is typically foilable by playing at "+".

The foil may not work if Red has a lot of space. For example, the following position is winning for Red (with Blue to move, and assuming "*" connects to the top edge), but Red needs at least the amount of space shown. If any one of the empty cells is occupied by Blue, the position is foilable.