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[Asellus]

Here is a puzzle I came across that I discovered can be solved using only a single advanced step. However, it's not so easy to spot.

[TKiel]

Nothing more advanced than naked pairs to this point, then 'the step', then nothing more advanced than hidden singles.

[nataraj]

could it be you guys are referring to a rather long chain linking r2c1 and r9c2?
Not sure I could call this a W-wing but it certainly does the trick by eliminating 2 from r1c2 and r7c2.

[nataraj]

as a slightly differnt approach, one could start coloring the '12' cells in box one and arrive rather swiftly at r5c8=1 => r9c8=1 => r9c2=2 and so on.

[Asellus]

Yes, a long XY-Chain is the one advanced step. There are at least two of them. (There's one that eliminates <6>s in Boxes 3 and 6.)

Nataraj,

Your coloring is not conventional multi-digit coloring because it involves several weak links. Rather, it is color marking the XY Chain implications. (In other words, it is identical to XY Chaining: your coloring reveals the XY Chain with <6> pincers I mentioned above.)

Also, I believe you meant to write "R5C8=8" and to mark the <6> as "rg" in R5C8.

If one first applies the X-Wing on <1> in C28, then conventional Medusa (multi-digit) coloring results in a "Medusa Wrap" that reveals this same contradiction:

[Johan]

I found a 9-cell xy-chain that solved the puzzle in one time

But like Asellus said, very hard to spot.

[Marty R.]

[Asellus]

[Johan]

This is the 9-cell xy-chain, eliminating <3> R2C5.

[37][79][96][68][81][12][21][12][23]

[TKiel]