Jeremy Gibbons Graham Hutton Thorsten Altenkirch When is a function a fold or an unfold? 2001 44 Electr. Notes Theor. Comput. Sci. 1 http://www.elsevier.com/gej-ng/31/29/23/73/23/show/Products/notes/index.htt#008 db/journals/entcs/entcs44.html#GibbonsHA01