@unpublished{upenn2009,
 author = {J. M. Siskind},
 title = {Automatic Differentiation of Functional Programs or Lambda the
          Ultimate Calculus},
 location = {University of Pennsylvania},
 date = 20,
 month = mar,
 year = 2009,
 url = {http://engineering.purdue.edu/~qobi/tweet-talks/upenn2009-slides.pdf}}