Box Line Reduction

When a specific candidate value in a row or column only occurs in one box, this value can be exluded from other cells in the same box.
This is the opposite of the Pointing Pair technique.
Sometimes a candidate within a row or column is restricted to one box.
Since one of these cells must contain that specific candidate,
the candidate can be excluded from the remaining cells in the box.
In the example on the right, we can use this technique in the left column of the bottom box. Since every candidate
value 4 in the left column is in the bottom box, one of these cells must contain a 4. Otherwise the column would be
without a 4. Therefore, all the yellow cells cannot contain a 4. These candidate values can be excluded.


