@Misc{ 
  ambkleled08,
  author = { Ambrosio, Luigi and Kleiner, Bruce and Le Donne, Enrico },
  title = { Rectifiability of sets of finite perimeter in Carnot groups: existence of a tangent hyperplane },
  journal = {  },
  year = { 2008 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/46/ },
  note = { cvgmt preprint}
}