I don't have any problem spotting the ALS, the difficulty I'm having is trying to figure out which candidates can be eliminated from which cells. I'm not confident that I understand the definition of "weakly associated" cells.

Take this puzzle for example (I realize simpler techniques could be applied to solve the puzzle)

- Code: Select all
`|---c1--|---c2--|---c3--||---c4--|---c5--|---c6--||---c7--|---c8--|---c9--`

-----------------------------------------------------------------------------

r1 | 238 | 689 | 269 || 368 | 4 | 5 || 7 | 23 | 1

---+-------+-------+-------||-------+-------+-------||-------+-------+-------

r2 | 357 | 67 | 567 || 1 | 36 | 2 || 9 | 8 | 4

---+-------+-------+-------||-------+-------+-------||-------+-------+-------

r3 | 238 | 1 | 4 || 9 | 7 | 38 || 5 | 23 | 6

===========================||=======================||=======================

r4 | 1 | 3 | 579 || 567 | 69 | 67 || 2 | 4 | 8

---+-------+-------+-------||-------+-------+-------||-------+-------+-------

r5 | 25 | 4 | 8 || 235 | 13 | 13 || 6 | 9 | 7

---+-------+-------+-------||-------+-------+-------||-------+-------+-------

r6 | 6 | 79 | 279 || 278 | 89 | 4 || 1 | 5 | 3

===========================||=======================||=======================

r7 | 78 | 678 | 67 || 4 | 2 | 9 || 3 | 1 | 5

---+-------+-------+-------||-------+-------+-------||-------+-------+-------

r8 | 9 | 5 | 3 || 78 | 18 | 178 || 4 | 6 | 2

---+-------+-------+-------||-------+-------+-------||-------+-------+-------

r9 | 4 | 2 | 1 || 36 | 5 | 36 || 8 | 7 | 9

.............................................................................

ALS A = r4c5, r6c5 {69, 89}

ALS B = r4c6 {67}

My flawed understanding is this:

1. A and B are linked by 6.

2. The unique values of each ALS can be eliminated as candidates from all other cells in the same unit as A and B except for the nodes of A and B.

3. The unit containing A and B is block 5.

4. 8 and 9 are unique to A and therefore can be eliminated as candidates for all cells in block 5 except for the nodes of A and B.

5. 7 is unique to B and therefore can be eliminated as a candidate for all cells in block 5 except for the nodes of A and B.

I know my understanding is flawed because having already seen the puzzles solution, I know that r6c4 is a 7 (which would have been eliminated in step 5 above).

I've been trying to use Bob Hanson's solver to reverse engineer the rule for which candidates can be eliminated but I just can't seem to grasp it. Could someone please explain the logic of which candicates can be eliminated from which cells. I'm not that savy with all the set and math theory so the simpler the terminology the better.

Thanks for your help