In the review of PencilPress Extreme Sudoku, 41 is a another bypass blow out, with two steep collapses. Tom Sheldon’s Master Class 50 hands you a hidden dublex that is a challenge to spot and verify. We’re reviewing the PencilPress collection for the first time, and updating the Master Class review of 2013.

Let’s say you are reading the basic trace of Master Class 50 and get to the NEhdx5.

Your grid copy looks like this:

Now its up to you to explain how SW5 can create a hidden dublex 5 in the NE box, and where it goes.

What’s a “hidden dublex”? Welcome to Sysudoku, and look in Sudoku Speak or The Guide.

Now while you’re trying to think,

here’s the bypass trace for PencilPress bottom right, page 41:

With the effects of C7 posted, E9 triggers the remaining collapse. This is not extreme Sudoku, but PencilPress is right in their cover page advice to older Sudoku fanatics like me: “If you play Sudoku daily, you will start to see improvement in your concentration and overall brain power.”

Now before confirming your answer to the hidden dublex problem, here is the bypass collapse of Master Class 50:

OK, now about that hidden dublex, the pencil mark 5’s lead the way from SW5 to NEhdx5.

That’s looking four steps ahead, but it’s a regular follow up in the bypass, not a search. One slink leads to another. The only trick is to see that N5 excludes 5’s from Cr5 as well as Wr5, forcing a 5 in Er5. That means the 5 in c9 has to be in NEc9.

Sysudoku posted traces abound with subtleties like this. Many of them, like this one, are revealed in box marking.

Next week, it’s PencilPress 49 (left) with the same result, but a different profile of bypass collapse. Master Class 60 (right) falls in line marking, due to an improved Sysudoku Basic. This preempts a regular XYZ-wing and three XY ANL from the earlier review. If your Basic gets there better, show me.