http://www.iai.uni-bonn.de/~jv/free-slides.pdf
http://daniel.yokomizo.org/2011/12/understanding-higher-order-code-for.html
http://arxiv.org/pdf/1107.1203.pdf
(Also in typeclassopedia Section 3.3)
http://hackage.haskell.org/package/free-theorems-seq
http://hackage.haskell.org/package/free-theorems-counterexamples