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Sudoku technique · Specialty reference · reference
3D Medusa
Two-color a graph of candidate nodes. Strong links in bivalue cells connect different digits, extending single-digit coloring.
How to spot it
Alternate colors over both two-location house links and bivalue-cell links.
The common mistake
A node is a digit-in-a-cell, not a whole cell. Conflicts can occur either within a cell or between equal digits in a house.
Learn this first
Coloring and bivalue links.
Further reading: sudokuwiki.org