Reference:
  Author: Stéphane Dray and Anne-Béatrice Dufour
  Title: "The ade4 Package: Implementing the Duality Diagram for Ecologists"
  Journal: Journal of Statistical Software
  Year: 2007
  Volume: 22
  Number: 4
  Pages: 1-20
  URL: http://www.jstatsoft.org/v22/i04
  eprint: http://www.jstatsoft.org/v22/i04/paper
