Reference:
  Author: Daniel Müllner
  Title: "fastcluster: Fast Hierarchical, Agglomerative Clustering Routines for R and Python"
  Journal: Journal of Statistical Software
  Year: 2013
  Volume: 53
  Number: 9
  Pages: 1--18
  URL: http://www.jstatsoft.org/v53/i09/
  eprint: http://www.jstatsoft.org/v53/i09/paper
