@unpublished{hertfordshire2005,
 author = {J. M. Siskind},
 title = {Taking Derivatives of Functional Programs: AD in a Functional
          Framework},
 location = {University of Hertfordshire},
 date = 22,
 month = nov,
 year = 2005,
 url = {http://engineering.purdue.edu/~qobi/tweet-talks/hertfordshire2005-slides.pdf}}