Here is a follow up on the advanced level More Extreme 4, with background looks at the bv map, XYZ map, and X-panel . There is a challenge to find a forcing chain XYZ alongside a regular one.

Entering the bv scan with the More Extreme 4 line marked grid of the last post, we were short of candidates or bv on every Sue de Coq or APE chute, but the XYZ map made up for it.

Here it is shown with several rejected hinges, and two live ones. There is a regular 347-wing and an irregular one constructed with a forcing chain. Both have victims. If you did not find them, jump back to the line marked grid, and take a shot, with the hinges and wings shown here. I’m placing the grid marking of these wings after the other eliminations, but don’t peek.

Going to the 3-panel after the wings, we catch either a swordfish or a complementary jelly fish, depending on where your net landed. They have the same four victims.

Now if you haven’t done it already, you can verify that the fish victims enable us to draw this XY rail, the first one available. On the grid, the slink ending 7-candidates are a toxic set, removing 7r5c3.

But neither that removal, nor the new bv it creates, seems to matter.

Instead, the more decisive result from the fish attack is the network of strong links among the remaining 3-candidates. The 3-chain analysis yields nothing more. Unfortunately, the octagonal loop is closed with two adjacent winks.

However the long slink chain is a strong asset in Medusa  coloring.

The blue/green cluster produces nothing immediately, but when the AIC hinges are added,an AIC chain is found for a strong link between blue 3r5c8 and 3r4c5, extending the cluster by making 3r4c5 green.

Did that one get by you? It’s just putting together what we already know. The chain says that if 3r5c8 is false, then 3r6c7, 9r6c1, 5r4c1 and 3r4c5 are all true. That’s the definition of a strong link! And slinks are the building blocks of Medusa clusters.

Please note, T&E worry warts, there is no assertion here that 3r5c8 is false. Nor is there any test of its falseness. There’s only a demonstration that a remote slink exists. Fortunately, that slink extends the cluster into a wrap. Two green 3s wind up in c5 and in the C box. For that reason, the green army is false, and the blue army, including 3r5c8, is true. More Extreme 4 collapses in a heap.

OK, you didn’t peek and you found at least one of the XYZ-wings. Right?

The regular orange 347-wing hinged at r4c5 eliminates 7r4c6.

In the burnt orange 347-wing hinged at r1c1 is something else. The 37 wing is sees the hinge 3 by forcing chain. One victim sees the toxic set by unit winks, but 7r5c1 needs a grouped forcing chain to bow out.

Show this to your Sudoku friends. So far the forcing chain XYZ-wing is a Sysudoku exclusive despite numerous posts demonstrating it.

That was fun, but I want you also to experience the misery of line marking one of Alary’s stingies. Not that number scanning is any easier. For this mission, should you accept it, the assignment is More Extreme 104. Your 17 clues have just arrived. In the sea of candidates, there is a lifeboat. If you lose it in the fog, I’ll send the Coast Guard next post.