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Simon Sporrong35,097 Points
Probably because the KaraokeMachine constructor takes a SongBook as a parameter. You can see it on line 18 in the video at 03:51 in the video.
So in order to instantiate a KaraokeMachine object you need to pass it a SongBook. What good is a karaoke machine without songs to sing in it? :)
Hope it helps!