 The lyrics to the song the choir sings in the Great Hall, "Double,  Double; Toil and Trouble..." are from  William Shakespeare (I) (qv)s "Macbeth" (Act 4, Scene 1). These  words are recited by three witches casting a spell to foretell the  murderous and treacherous Scottish kings future. As the story goes,  Shakespeare "borrowed" these lines from real witches, who in turn  cursed the play, which is the reason why the word "Macbeth" is  considered bad luck if uttered in a theatre.