Unintentional mezzo joke

Karin Dreijer Andersson and Olof Andersson, the Swedish electro-pop duo known as The Knife, are introducing their opera "Tomorrow, in a Year" in London.

"I didn't have any knowledge of opera," Karin tells The Guardian's Priya Elan, and goes on to prove it by saying the piece is scored for "a mezzo-soprano, an opera singer and an actress" . . .

http://www.guardian.co.uk/music/2010/jul/28/knife-opera

. . . bringing to mind the old joke about a male vocal quartet being composed of "three men and a tenor."