Empty Rectangle
Inside a box, a digit's candidates gather into one row and one column. Combined with a strong link outside, the crossing cell can be cleared.
How it works on this board
Inside the box, 7 is confined to one row and one column
1
6
9
5
4
7
3
8
2
6
1
4
8
6
7
6
4
2
8
5
2
8
4
2
5
9
3
2
1
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