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`*-----------------------------------------------------------*`

| 1 458 2 | 6 57 9 | 3 48 478 |

| 58 589 6 | 45 457 3 | 2 1 789 |

| 7 49 3 | 1 8 2 | 49 6 5 |

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

| 6 35 1 | 9 345 45 | 8 7 2 |

| 58 358 7 | 2 1 6 | 459 3459 49 |

| 4 2 9 | 8 35 7 | 1 35 6 |

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

| 9 7 8 | 45 6 1 | 45 2 3 |

| 3 1 5 | 7 2 48 | 6 489 489 |

| 2 6 4 | 3 9 58 | 7 58 1 |

*-----------------------------------------------------------*

Firstly, I want to apologize if the notation I use is non standard. I haven't quite figured out the proper way to express XY chains.

4-(r3c2)-9-(r3c7)-4-(r1c8)-8-(r9c8)-5-(r7c7)-4

This is a discontinuous XY chain of 5 bivalue cells, starting at r3c2 and meandering to r7c7. It should allow the exclusion of 4s in cells that see both r3c2 and r7c7.

But.. this would mean that the 4 in r3c7 is excluded. r3c7 is part of the chain. Is the exclusion still valid?

If the exclusion is valid, and r3c7 is forced to 9, does this solve the other cells in the chain? r3c2=4, based on completing row 3. But does it also lead to r1c8=4, r9c8=8 and r7c7=5?

Those are all the correct values for the cells, but does the chain prove it? Does it even prove the exclusion of the 4 in r3c7?