This is the last of the Weekly Extreme Competition Sudoku originally posted starting 3/31/2015. Reviews were designed originally to illustrate trial methods appropriate for the weekly deadlines of the competition.
In 2023 the trial methods were replaced by methods typical of, and generally in order of Sysudoku posts.
The boxline removal 1r5c5 is due to r6 1’s being confined to the C box.
A double hidden unique rectangle comes later in line marking. If 1 is is true in either order, 9’s are confirmed in adjacent corners and 1 is forced in the opposite corner. If this is in the solution, the 1 and 9 can be reversed for a second solution.
In the same grid area, a second hidden rectangle. If 5r5c2, the crossing 1 – slinks force 1’s in the adjacent corners, and the 5-slink forces 5 in the opposite corner.
Then 1r2c6 is removed by a sashimi 1-wing, or the blue/green trap.
Next, on our request, Beeby finds several ALS_XZ that depend on ALS of many cells and values, and therefore would require the systematic ALS mapping outlined in March and April of 2021.
The first example combines a bv with a 5-cell, 6-value ALS.
Next, two box ALS with 3 and 5 cells,
and column ALS of 5 and 3 cells.
More easily spotted is this one, with a bv and a 3-cell ALS.
AIC Building starts with a 5-chain between two bv.
After a violet/tan cluster is added, two bv and a 6-chain brings S9, and a complete XY ANL brings E3.
The 3r6c8 removal creates naked pair r6s57 for C1, C8 and S5.
A long, but simple ANL removes 7r2c8 to make tan and orange opposing colors, merging two of the clusters.
In r3, green and orange 1 conflict for an blue or red bridge.
Now NW3 will turn 1r9c1 red, and the r9 red and green 1 conflict is a blue and orange bridge confirming blue.
There’s not much left to fill in.
Systematic Sudoku 