- Code: Select all

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| 69 38 2 | 5 3689 1 | 7 46 346 |

| 67 1 5 | 4 36 37 | 9 2 8 |

| 79 4 38 | 368 2 79 | 136 16 5 |

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

If r1c2=3, then r3c7 must be 3. If r3c3=3 then r1c9=3. Either case will eliminate 3 as a candidate from r1c5 and r3c4. So even though I don't know the location of 3s in boxes 1 and 3, I can eliminate two candidates from box 2.

I haven't done a detailed study of colouring yet, but I'm guessing this might be an application of it. Is that correct, or is this some weird variation of X wing (I also haven't learned XY wing).