total_duration([]) => 0
total_duration([shape_of_you,good_4_u]) => 7
total_duration([unknown_song]) => false
make_playlist(15,16,3) =>
  [[happier_than_ever,rocket_man,skyfall]]
make_playlist(10,11,2) =>
  [[happier_than_ever,rocket_man],
   [happier_than_ever,skyfall],
   [rocket_man,skyfall]]
make_playlist(11,12,2) => []
