It’s mostly in Basic with Super-Tough v. 5, b.4, # 5. The post of March 26, 2013 reports a long XY chain, followed by two remarkable irregular XYZ-wings, then two finned swordfish. However, this update shows that the generous bv field can be exploited more immediately.
Here is the grid as line marking. Is completed. You notice first the large number of bi-value cells, the bv. Now scan over the field to see how well they are connected. There’s an extensive XY Railroad in there for sure.
But you can throw a coloring cluster on the bv field much faster than drawing a railroad, and if the cluster does nothing, still it won’t interfere with the bv scan, or the railroad.
Do that while I report the Basic trace. Starting with blue 1r1c3 cross every strong link, changing the color on every one. A slink requires exactly two candidates of a value in every row or column, or exactly two candidates of two values in a cell.
Here’s exactly how the two 4-fills came up in the bypass, and how a hidden dublex claimed 6 more cells in box marking. Also, the trace shows that three rows were left when candidates were all identified. Did you know that’s what those chicken scratches were about? You can follow every move, find my mistakes, and question my tactics. We both win.
You don’t need to read explanations on the traces page. Just follow along with your grid, do what comes to you and see if and why every move is justified, and how those moves are described.
Now back at your colorful grid, what did you do for 3r8c4? 3r3c4 and 3r7c6 say “color it green” while 3r8c7 says “color it blue. It’s not majority rule here. It’s that blue or green is true, and 3r8c4 sees both blue and green 3’s. It is removed, leaving a blue clue. All blues are true, and all greens false.
Next week its Super-Tough v.5, b.5, #5. Get the puzzle at krazydad.com.
Systematic Sudoku 