Box marking gets to shine in the redo of Super-Tough, volume 5, book 7, #5.
Here is the grid after the relatively modest bypass. The fill strings show where 4-fills were examined, but only one was found.
In this snapshot, box marking is just beginning to pay off, as the C box is claimed and the 23 pair slice up the center 2 candidates.
Below is the trace up to that point.
Here’s the entire 9’s box marking and the line marking.. It’s my best example of box marking slinks taking space and generating clues. Typically box marking generates no clues.
In the final line marked grid, I could color now, but let’s see if some old friends show up first.
On the left, a hidden unique rectangle. We look for pairs on rectangle corners. The slinks say that if UR candidate 9r6c2 is true, it’s corner 6 is false, placing 6 in both adjacent corners. The bv corner then forces the opposite corner to 9, and that solution on the rectangle can interchange UR partners and become a second solution.
On the right, an irregular 139-wing. One of the hinge or wing Z candidates is true. 9r6c8 sees the hinge and one wing via column 8. It sees the other Z via 9-chain.
Then on our regular visit to the railroad station, two XY ANL confirm 1r9c3 and 9r9c5. That’s enough for a quick collapse, but . . .
coloring gets the same results easier and faster, trapping 9r1c3 and 3r1c5 and wrapping green in c4 and c5. A consistent result.
For next week, look up KrazyDad Super-Tough v.5, b.8, number 5.
Systematic Sudoku 