Seeing that many people struggle to get their heads around the pattern, I'll explain the way I found it (it's a bit different from shye's intended way).
The first thing I noticed when looking at the puzzle (after filling in 9r4c2) was the pattern of givens in r19c19b1379.
If no digit were given both in the lines and the boxes, we would get something very similar to an SK-loop (only the givens that usually are in the corners would now be in the minilines in b2468).
Since many digits do appear in both sets, I decided to at least check out 2 and 8 – they appear only in one set, twice each and they're given in the same boxes.
In r1 and c9, one of them can take the intersection, the other must appear at least once in r1c5, r5c9. By symmetry, there is a 2 or 8 in r5c1, r9c5.
Now I looked for ways to limit the number of 28s in these cells and it turned out that r5c5 did exactly that. There cannot be more than two of them, one in r5, one in c5.
This is enough to prove the pattern rank0, so if you just remember the links and carry out the eliminations, you're fine. Worst-case scenario, you forget to do the eliminations in r1c9 and r9c1.
I hope someone will find this explanation helpful and maybe spot the pattern next time if they ever encounter it in another puzzle (which maybe could be called Shellraiser
).
Marek
