This post traces the bypass finish of Classic 240 and presents the review table for James Forest’s Classic Sudoku. Then it continues the update review of Manuel Castillo’s Only Extreme Sudoku with Only Extreme 261. Demonstrating a solution without trial, Sysdudoku assigns Only Extreme 261, along with other Castillo Only Extremes and KrazyDad Insanes, and Classic 144, a new level of difficulty introduced in this post, the AIC-net level.

With Sudoku experience – or the interest to work through the Basic Sudoku pages of the Guide – you can read the Basic traces of Sysudoku posts and observe its humanly efficient manner of solving basic level puzzles, while preparing the grid for advanced methods. It’s done by observing how your own solving steps are ordered and expressed in the trace.

Here is the folded trace for the last pre-selected puzzle of the Classic Sudoku review, Classic 240.

The review table for Classic Sudoku reveals a surprising range of difficulty level. Six of ten are Basic level. Four collapse in the bypass, while one is beyond advanced, requiring an AIC building project.

The book contains no notes about the collection, so we don’t know whether this diversity is intended or not. The cover description of “Very Hard” just doesn’t fit. Most collections are much more consistent on a level of difficulty.

Turning now to Only Extreme 261, here is the line marked grid, with a hidden unique rectangle, Type 2b that I missed in the post of 11/28/2017, Pattern Coloring Not-Both on Only Extreme 261.  The UR is signaled by the naked pair and a slink from one of its corners. If 8r7c3 is placed, the rectangle corner placements can be interchanged for a second solution.

Since trials was an issue in the Only Extreme review, it is relevant that NEr1 is a single alternate Sue de Coq, whose alternate term 8(27+72) solves 261 outright. The case was not closed with this trial, because there was hope for examples of not-both trial coloring extensions.

In the original post, a 7-chain ANL and an irregular 562-wing hinged at r4c8 are the only eliminations ahead of AIC building. In a failing session of AIC building, the not-both extension of an AIC slink was seen to be equivalent to a color trial of a pattern based cluster found earlier on the 7-panel. With help from a pattern analysis trap on the 8-panel, the trial turned out to be easy and decisive. But this success left open whether or not OnEx 261 could be solved without a trial.

Putting the question to Sudokuwiki in this update, the solver catches the 7-chain ANL, and moves directly to AIC building, demonstrating this AIC confirming ANL,

followed by a relatively easy boomerang  (left below),

and the duplication of the irregular XYZ effect by an AIC boomerang.

After the grouped ALS boomerang here, Sudokuwiki stalls, leaving only the limited blue/green cluster.

The 5-panel eliminations have left a single olive and three pink patterns.

Fortunately for trial avoidance, the three orphans include the blue 5r9c4, confirming green.

As the grid opens up, an irregular 152-wing brings more bv.

Sudokuwiki is revived by these results, and puts together an AIC  double grouped confirming ANL.

After the  follow up below, the red/orange cluster of the original post is  overwhelming.

 Red is wrapped in c8 and orange mops up.

Next is the update on Only Extreme 304 originally solved by six trials over two posts, starting 12/05/17.  With help from the Sudokuwiki solver, it is solved without trial.

Puzzles requiring extensive AIC building are deserving of a difficulty level to themselves. Reserving the “extreme” level for those requiring trials, Sysudoku will be calling the new level the AIC-net level.