(a) the five-vertex running example:
   entries Dijkstra got wrong: 0 out of 25

(b) a four-vertex graph built to expose the flaw:
   delta(1,2): Dijkstra says 1, the truth is -1  <-- WRONG
   delta(1,4): Dijkstra says 2, the truth is 0  <-- WRONG
   entries Dijkstra got wrong: 2 out of 16

Moral: on (a) Dijkstra happened to be right, and that is worse than
being wrong, because it invites you to trust it on (b).
