Learn / Specialty reference
Sudoku technique · Specialty reference · reference
Sue de Coq
A constrained box-line intersection combines with disjoint supporting candidate sets in the rest of the line and box.
How to spot it
Study candidate counts and how the two supporting groups reserve digits.
The common mistake
This is not any arbitrary pair of ALSs. Use the full counting conditions in the worked reference.
Learn this first
Subsets and ALS.
Further reading: hodoku.sourceforge.net