6 vertices, 14 edges
  0 -> 1
  0 -> 2
  1 -> 0
  1 -> 3
  2 -> 0
  2 -> 3
  2 -> 4
  3 -> 1
  3 -> 2
  3 -> 5
  4 -> 2
  4 -> 5
  5 -> 3
  5 -> 4
