This method can be used when a great number of cells contain only two possible candidates.
in choosing one of the candidates in a starting cell, one forms a chain of choices wich results in the suppression of a candidate in a given cell.
If the other candidate is chosen in the starting cell and one ends with the suppression of the same candidate, it can be excluded safely.

	Starting with B2, if 3 is true then one has a chain of consequences (as following in red): B2 : 3, D2 : 5, E3 : 7, E5 : 8, A5 : 5, one can thus exclude 5 from A4 If it is 2 wich is true in B2, then we have (as following in green): B2 : 2, B4 : 5 and 5 can be excluded from A4. In any case, 5 can be excluded from A4.

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