This begins a brief review of Manuel Castillo’s Only Hard Sudoku, preliminary to reviewing his more recent Only Extreme Sudoku. Both books are wordless collections of 400 puzzles, with identical formats.

Thanks go to officers, members and guests of the Fairlawn Ohio Seniors Club, for a warm welcome. We walked through the start of  basic Sysudoku on a newspaper 4-star, and learned bypass slink marking, 3-fill rules, and importantly, how to read a Sysudoku basic trace. The audience started a handout grid, and have a full basic trace to finish the puzzle.

Now to Castillo’s Only Hard collection,  here’s a Sysudoku review table with preselection of 2, 44, 86, . . . , 380. The Only Hard range in Sysudoku survival from bypass to line marking close, with one brief glimpse at the advanced level.

Details of review table notations are on the Reviews page.

We trace Number 128 to show an untypical hidden pair.  I spotted it in my systematically plodding way, in the line marking close, but reading back in the trace, I realized it could have been spotted in box marking. Starting the 8: list, and adding 8 marks to the r9 naked triple, the 3 and 8 clues in the East box limit placement of the 38 partners to r2c8.

The usual hidden pair spotting aid, the naked triple, has not arrived. Here we depend on the definition of a hidden subset: n numbers in only n cells.

If you spotted it there, congrats! My revised trace starts the collapse right there, with the pair. Here’s the whole thing:

Next time, checkpoints on 254, the bypass fatality on the left, and holdout 338, on the right.

After a timely multiple solutions report, the review of Only Extreme Sudoku continues.