In addition to Sue de Coq and Unaligned APE, there’s another reason to watch for almost locked sets (ALS), on the bv scan and after eliminations. And again, it’s because the ALS that loses a value becomes locked, with all of its remaining values toxic.
If you can, imagine two egg shaped ALS sharing four values, depicted here as colored areas. One set, the black candidates all see each other. Being ALS, both sets of 3 cells have candidates of 4 values. Since the Sudoku placement administrator does not allow solution candidates to see each other, only one of the two ALS can have a black solution value. That means that at least one of the ALS will give up its black values and will be locked, meaning it will have a blue, a red and a green placement. It doesn’t matter which ALS is locked. Any outside candidate seeing all candidates of an uncommon value in both ALS is summarily dismissed.
In real life, it doesn’t work quite that idealistically. ALS are plentiful, but to find these toxic sets with a human scale search, you have to look in just the right places. In his introduction to Almost Locked Sets in The Logic of Sudoku, Andrew Stuart illustrates a very “right place”.
Andrew’s illustration, Figure 29.1 comes after this action on the line marked grid, which Andrew left to the reader.
It’s a hidden pair on c8 and a finned 5-wing, that eliminates 5r6c5, permitting . . .
. . . a four valued ALS with single candidates of two values, near a bv of those same values. Those conditions make “seeing all . . . in both” easy. Sure enough, the 1’s see each other, forming a “restricted common”. The other shared, but uncommon value 5, is locked.
The 5 victim sees all of the 5’s in both ALS. Sounds hard, but here its easy. If the bv gets the 1, then 5r1c5 is true. If the c5 ALS gets the 1, then the victim sees a 5-clue at r6c3.
The combination of a bv and a larger ALS is appropriately labeled an ALS-XZ. Conditions are stricter than the BARN, but it covers more. Here, in fact, the two ALS are not in a bent region. On the other hand, many BARNS are also ALS_XZ similar to this one.
Like many Sudoku techniques, knowing why they work doesn’t tell you how to spot effective ALS_XZ among the candidates on the busy grid. Here is a Sysudoku technique for directing your attention to promising cases, known here as ALS partnering.
By this prescription, how would it find the Andrew’s ALS_XZ? Looking for an RC with 15 in r6c3, you might have rejected 5 in r1c3 (no 1), r2c3 (both 1 and 5), r4c3(no 1), r5c3(no 5), and r6c2(no 1). Now partnering with r6c5 on common 1, you’re adding c5 cells for an ALS including 5’s and avoiding 1’s.
With r6c518 first, you would be looking for 8’s and avoiding 5’s. Finally, with 15 in hand, you could be ALS building in the NW box for 5’s, avoiding 1’s or for 1’s, avoiding 5’s.
Now for some homework, link back to the post introducing the ALS toxic set, and verify that all of the ALS toxic sets could be found by ALS partnering. You can also practice ALS partnering with the very next example from Stuart’s The Logic of Sudoku, which has a 2 cell, 3 value ALS as a first ALS.
Transcribing Logic Figure 29.2 into Sysudoku slink marking, the blue ALS is seen to have two single candidate values, 1 and 5. Building out along r2 from a possible grouped restricted common, we add the bv 25 cell without increasing the number of values, to get an elimination partner ALS.
If there’s no immediate collapse, we might realize that the said grouped restricted common also works for another partner.
For those just arrived from the bent family guide page, this second ALS toxic set example is also a 5-set BARN or BNS1.
Lest your scan be confined to neighboring lines, here’s one last ALS_XZ, found by the ALS_XZ sleuth of Sysudoku, Gordon Fick. It’s #200 from the review of More Extreme Sudoku by Antoine Alary. Another grouped restricted common, and amazingly enough, another BARN/BNS1, and starved crocodile ready to eat my words.
I should mention one subtle point on the ALS_XZ that has tripped me up. The restricted common members all see each other, but the restricted common is not toxic. Do not remove 6r9c5. Even though one ALS must go without a 6, the common set doesn’t necessarily get one, unless it’s also a slick.
OK, are you back? I have one more startling fact to share. Suppose there are two values in restricted common between the two ALS.
That is possible. Here is Fick’s double common ALS_XZ on More 200, occurring right after the crocodile above. With double commons, every uncommon value in each ALS is locked. Double independent smaller sized toxic sets. Why? Each ALS loses a value to the RC and becomes n values for n cells. Now 6r9c5 is shut out. But not 8r1c8. The common values are now 8 and 7. Cool?
The natural time for ALS partnering is the bv scan, but it also pays to watch for opportunities to develop with eliminations.