(a) the running example (edges of weight -4 and -5, but no negative cycle):
   diagonal after the triple loop:    0   0   0   0   0
   negative-weight cycle? no

(b) a graph whose cycle 1 -> 2 -> 3 -> 1 has weight 2 + 3 + (-6) = -1:
   diagonal after the triple loop:   -1  -1  -2
   negative-weight cycle? YES

(c) a graph whose cycle 1 -> 2 -> 1 has weight 1 + (-4) = -3:
   diagonal after the triple loop:   -3  -6
   negative-weight cycle? YES
