--- bifunctor formalism for fold
http://blog.ezyang.com/2012/10/duality-for-haskellers/
https://kseo.github.io/posts/2016-12-12-unfold-and-fold.html http://www.cs.ox.ac.uk/jeremy.gibbons/publications/hodgp.pdf http://www.cs.ox.ac.uk/jeremy.gibbons/publications/cwh-slides.pdf
http://www.prg.nii.ac.jp/course/2013/springCourse/source/lecture1.lhs
http://www.cs.nott.ac.uk/~pszgmh/when.pdf
https://braincrater.wordpress.com/2008/07/07/fixed-point-datatypes/
--- a more general setting , F-algebra
https://www.schoolofhaskell.com/user/bartosz/understanding-algebras
http://homepages.inf.ed.ac.uk/wadler/papers/free-rectypes/free-rectypes.txt
http://web.cecs.pdx.edu/~sheard/course/AdvancedFP/notes/CoAlgebras/Code.html