This heuristic differs from the earlier ones mentioned in that it does not seek to eliminate a single possible value. The basic idea is simply to examine all the remaining possible rows and columns that each of the values from 1 to 9 can occupy. If only one of the available row/column combinations is possible for a given value, that row/column combination can only hold that value.
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Message:
Cell (1,4) currently contains possible values [4,6,7,9]. However, the values other than 4 are irrelevant. (1,4) can only contain 4 because every row other than 1 already has a 4 and every column except 4 has a 4. Hence, we can set (1,4) to 4 (highlighted in gold). |