[Help-glpk] Chess Problem

[BillGrieves]

Hello,
Does anyone know how to write a function to sum the diagonals of a matrix in
glpk? It is simple to sum the rows or columns* but I cannot figure out how
to write it for diagonals.

*sum{i in xxx} y[i,j]

For anyone interested, this code needs to provide the solution for the
maximum number of bishops on a chess board without any bishop being under
attach. The matrix will consist of 0's for empty spaces on the board and 1's
where a space is occupied by a bishop. So the sum of both diagonals in which
a bishop lies must be 1. Any advice would be much appreciated.

Thanks,

Bill
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