Assuming a board in the style
b = [
nil, nil, nil, 3, 9, nil, nil, 1, nil,
5, nil, 1, nil, nil, nil, nil, 4, nil,
9, nil, nil, 7, nil, nil, 5, nil, nil,
6, nil, 2, 5, 3, nil, nil, 7, nil,
nil, nil, nil, nil, 7, nil, nil, nil, 8,
7, nil, nil, 8, nil, nil, 9, nil, 3,