@misc{Cas15,
  author = {G. Castagna},
  title = {Covariance and Contravariance: a fresh look at an
    old issue (a primer in advanced type systems for
        learning functional programmers)},
  year = {2015},
  note = {Unpublished manuscript},
}

@phdthesis{Fri04,
  author = {A. Frisch},
  title = {Théorie, conception et réalisation d'un langage adapté à XML},
  school = {Université Paris Diderot},
  year = 2004,
  month = 12,
}