Sudokuwiki Finishes ultrahardcore 91

This post shows the Sudokuwiki solver path to the coloring finish of trial wounded ultrahardcore 91. Last week it was with Phillip Beeby’s solver. You could use either solver as a model, depending on how you would rather spend your searching effort.

Sudokuwiki advanced begins with two trial won clues, two X-chain ANL, and the same small blue/green cluster.

Next is this AIC confirming ANL. Beeby claims this clue prior to AIC building by eliminating its slink partner 2r7c3 by ALS_XZ.

This APE is also duplicated by a Beeby ALS_XZ.  The two cells of the the aligned pair cannot contain a combination of values they see in an ALS.

Sudokuwiki notes present this next grouped AIC boomerang as a digit forcing chain from any candidate along the chain to 6r5c9. More accurately, it is a boomer from 7r5c9. Here is a little pop quiz on recent posts. Can you present it as a Beeby 1-way? Yeah, it’s a stretch.

Now it’s an easy ANL,

and after a little follow up,

a rarity, a regular 148-wing.

Next, the first of two BARNs, or in Sudokwiki notes, a WXYZ-wing. It’s a bent region naked set, with one value bent, the others in one unit.

Another rare entity in ultrahardcoreville, the simplest AIC, an XY ANL.

Then a second BARN, this one missed by Sudokuwiki.

Almost there, we get a hidden UR, type 1. The slinks opposite the extra free corner carry 8 to two corners around it.

Sudokuwiki’s  triple  grouped ANL is impressive,

but a regular Sysudoku Sue de Coq

SEc7 = 1(2+9)(7+8)    gets the removal and more.

SEc7 must contain one from each alternative, and a 1. From the SEbox remainder, only the the bv 18 can supply a missing 8.

The coloring cluster expansion overtakes one XY chain, but a second one keeps it going.  We’ve reached the same state of expansion seen in the previous post on Beeby’s solution.

At the finish, the blue wrapping cluster demonstrates how close uhc 91 comes to two solutions. This form of multiple solution is tough on logical solving methods. It doesn’t help with intuition either.

Next week continues the Heine ultrahardcore review with the fourth puzzle, uhc 135.