@unpublished{nips2016a,
 author = {B. A. Pearlmutter},
 title = {Automatic Differentiation: History and Headroom},
 locxation = {NIPS Workshop on The Future of Gradient-Based Machine
              Learning Software},
 date = 10,
 month = dec,
 year = 2016,
 url = {http://engineering.purdue.edu/~qobi/tweet-talks/nips2016a-slides.pdf}}