Step two: Normalize the relationships so duplicate statements are removed and they all show the same relationship, in this case "before":

Blitzen < Dancer.
Blitzen < Donder,
Blitzen < Vixen
Comet < Donder
Cupid < Donder
Cupid < Blitzen
Cupid < Comet,
Cupid < Dancer.
Cupid < Rudolph
Cupid < Vixen.
Dasher < Donder
Dasher < Blitzen.
Dasher < Dancer
Dasher < Vixen,
Donder < Dancer
Prancer < Blitzen.
Prancer < Cupid,
Prancer < Donder
Prancer < Comet
Prancer < Dasher
Prancer < Rudolph
Prancer < Vixen
Rudolph < Dancer
Rudolph < Dasher.
Rudolph < Donder,
Rudolph < Comet
Rudolph < Vixen
Vixen < Comet.
Vixen < Dancer
Vixen < Donder