Comment by soveran

For an alternative solution, describe the directed graph by listing the nodes and using whitespace to represent the arcs:

  $ cat riddle
  Vixen Rudolph
  Vixen Prancer
  Vixen Dasher
  Dancer Vixen
  Comet Vixen
  ...
  Vixen Dasher

Then use tsort:

  $ tsort riddle
  Dancer
  Donder
  Comet
  Vixen
  Blitzen
  Dasher
  Rudolph
  Cupid
  Prancer