[Illustration]
It will be seen in the illustration how the prisoners may be arranged so
as to produce as many as sixteen even rows. There are 4 such vertical
rows, 4 horizontal rows, 5 diagonal rows in one direction, and 3
diagonal rows in the other direction. The arrows here show the movements
of the four prisoners, and it will be seen that the infirm man in the
bottom corner has not been moved.