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Sudoku technique · Specialty reference · reference
ALS-XY-Wing
A three-ALS chain uses different restricted common digits at its two joins. A shared endpoint digit supplies eliminations.
How to spot it
Replace the single cells of an XY-Wing with properly linked almost locked sets.
The common mistake
All occurrences of the endpoint digit matter; ordinary cell-wing visibility is insufficient.
Learn this first
ALS-XZ.
Further reading: hodoku.sourceforge.net