@inproceedings{ aspinall94subtyping,
    author = "David Aspinall",
    title = "Subtyping with Singleton Types",
    booktitle = "Computer Science Logic ({CSL}), Kazimierz, Poland",
    publisher = "Springer-Verlag",
    pages = "1--15",
    year = "1994",
    url = "citeseer.nj.nec.com/aspinall95subtyping.html" }