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- Consider a digit X. Suppose the leading diagonal contains only two cells that has X as a candidate, call them A and B. Suppose the non-leading diagonal contains only two cells that has X as a candidate, call them C and D. If A and C are peers, then we can eliminate X from any cell that is seen by both B and D.
(X): B = A - C = D
Double crossover is a special case of Triangular Connection.