This post whittles on a tough grid, duplicating results of the two solvers by different means. When the solvers stall, we find a Single Alternate Sue de Coq for readers to try.
Sysudoku Basic brings four clues and slinks, but and reserves two cells for two values in ultrahardcore 1.
We start with a very heavy grid
My two solvers start differently, both on the X-panels. Sudokuwiki finds a 4-chain ANL. This trips a Sc4 boxline for a second clue.
Thinking to pick up any X-wing l might have missed. I asked Beebe for fish, and got this corresponding finned column 4-wing. The victim is in the fin box, so it sees the fin r8c4.
It happens again, when Sudokuwiki knocks out this grouped 7-chain ANL,
and I ask Beebe for the next fish. I get a finned jelly, in rows, to do the same removal.
Asking the question slightly differently, I get a finned 7-wing in different columns, with the same fin, and the same victim. I wouldn’t have thought it possible.
Will it work one more time? Here is Sudokuwiki’s intensely grouped 9 chain ANL, which it repeats to get both removals.
And here is Beeby’s corresponding finned swordfish.
Beebe closes down its humanly practical tools shop with this 1-way, when the slink chain started with a false 1r9c8 gets into r9c9 to erase 1r9c9. It’s also erased by a true 1r9c8
I try to extend my tiny 7 cluster with lite coloring, but there are no blue lite candidates for traps with my four green lite ones. Then freeform analysis failed.
NWc1 is a possible Single Alternate Sue de Coq in NWc1.
It’s up to you to read the Guide page on SASdC under Trials, set up the trial, and determine what you gain if the trial fails.
Systematic Sudoku 