generated at
Codensityモナド
http://myuon-myon.hatenablog.com/entry/2014/11/03/183032
https://en.wikipedia.org/wiki/Codensity_monad
https://ncatlab.org/nlab/show/codensity+monad
http://hackage.haskell.org/package/kan-extensions-5.2/docs/Control-Monad-Codensity.html