This post checkpoints Ardv2 78, the second review puzzle in A.D. Ardson’s Sudoku Very Hard collection. The puzzle skates by basic and the bv scan, but is tamed on the X-panel.
Along with regular collection reviewing posts, I’m revising the first year’s posts 2011-2012 to bring graphics, tracing and solving examples up to date.
The naked triple occurs on the closure of columns.
On the update, Beeby finds a very obscure hidden unique rectangle.
The next point of interest in Ardv2 78 is its remarkable resistance to the bv scan, with such a rich bv field.
. Here almost all of the duplicated candidates are in the same unit, and the corresponding slinks call for the X-panel and the crayons.
On the X-panels, a 4-panel slink chain allows itself to be interpreted as an AIC, for another removal.
Then comes a Sashimi 8-wing with a double fin.
On the first cluster, the 4- chain removal is duplicated and a second 4-chain becomes a coloring trap.
The second cluster makes a shortcut to a second removal, across clusters. That allows a third 4 ANL to start at 4r7c8.
The expansion slinks red and green 4-candidates in r7 and the SE box, merging the clusters. The merge brings two more traps, and a green wrap in c8.
The extensive cluster tells you something about why 78 is “very hard”. A large region of the grid is dominated by blue/green bv. This region can’t distinguish between two solutions.
Did an Amazon drone fly in the window with your volume 2 of 400 Ardson very hard Sudoku puzzles yet? If not, here is a copy of Ardv2 118 for next week.
Systematic Sudoku 
Maybe I’m just being dense, but where’s the c1 3-slink? You have rows 6, 8, and 9.
Not you, Dove. Me. Just delete that grouped 3-chain and put the victims back in. I’ll rework it from there. Thanks again.