“…Up to exchanging B 3 and B 4 and B 5 and B 6 , we can suppose that a 1 ≤ b 1 and a 2 ≤ b 2 . One computes that in order for the first rank 4 minor to be 0, we need that the quadruple (a 1 , b 1 , c 1 , d 1 ) is in the following list (0, 8, 8, 0), (1, 7, 7, 1), (2,6,6,2), (3,5,5,3), (4,4,4,4).…”