Complex Crosshatching
Confine a digit's candidates in a row to one box, eliminate the digit from the other rows of that box, and leave only one place for it in another column.
How 2 is placed
Row 7 confines 2 to the bottom-middle box, removing 2 from the box's other rows
2
9
4
9
3
1
9
3
2
8
4
9
6
8
2
8
6
9
6
9
5
4
8
7
3
8
7
2
2
6
9
4
9
4
7
4
2
9
2
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