@unpublished{purdue2009,
 author = {J. M. Siskind},
 title = {Automatic Differentiation of Functional Programs and its use for
          Probabilistic Programming},
 date = 30,
 month = mar,
 year = 2009,
 url = {http://engineering.purdue.edu/~qobi/tweet-talks/purdue2009-slides.pdf}}