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2. Sudoku Solver, Scanraid Ltd. http://www.scanraid.com/AdvanStrategies.htm
3. Maude: Specification and Programming in Rewriting Logic;Clavel;Theoretical Computer Science,2002
4. Bertram Felgenhauer and Frazer Jarvis, Enumerating possible Sudoku grids, Univ. Sheffield and Univ. Dresden, 2005 http://www.shef.ac.uk/~pm1afj/sudoku/sudoku.pdf