@Article{ 
  kirmag01,
  author = { Kirchheim, Bernd and Magnani, Valentino },
  title = { A counterexample to metric differentiability },
  journal = { Proc. Ed. Math. Soc.  },
  year = { 2003 },
  volume = { 46 },
  pages = { 221-227 },
  URL = { http://cvgmt.sns.it/paper/1604/ },
  note = { cvgmt preprint}
}