@unpublished{uc2008,
 author = {J. M. Siskind},
 title = {Automatic Differentiation of Functional Programs or Lambda the
          Ultimate Calculus},
 location = {University of Chicago},
 date = 8,
 month = apr,
 year = 2008,
 url = {http://engineering.purdue.edu/~qobi/tweet-talks/uc2008-slides.pdf}}