Sur le pont d'Avignon is a tune sung by Nancy Wagner beginning with chapter six until chapter 16.
It is a 15th century popular song about dancing on a bridge. In chapter 6, Nancy only sings the first two verses:
"Sur le pont d'Avignon / L'on y danse, l'on y danse..." and cuts off.
In chapter 16, she switches to a macabre version:
"Sur le pont d'Avignon / Everyone died, they all died"
She is mispronouncing the song by saying "la pont" and "on y danse" in stead of "le pont" and "l'on y danse".