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 XY-Chain Strategy
XY-Chain Strategy

Y-Wing chains are actually part of a larger family of chains of bi-value cells that’s rather like a platted thread.  Numbers intertwine from one end to the other.  If the two ends contain a candidate in common then we can show that one end or the other must contain that number and any cells seen in common cannot.  We don’t have to restrict ourselves to identical pairs.

Let’s inspect a chain just as a line of numbers:
[3/9] – [6/9] – [3/6] – [3/6] – [4/6] – [3/4]
All these cells will only ever have one of two possible solutions:
3 – 9 – 6 – 3 – 6 – 4
Or            9 – 6 – 3 – 6 – 4 – 3
If any number in a cell with two possibilities is true a cascade effect takes place along all links forcing them to certain values.  We can see the chain described above on the Sudoku board below.  The ‘pincer’ ends of the chain are in dark shades.

Fig. 23.1

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