For an alternative solution, describe the directed graph by listing the nodes and using whitespace to represent the arcs: $ cat riddle Vixen Rudolph Vixe

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