From: o.jasper on
http://ojasper.nl/Blog.html#Type%20calculation

Hope i wrote it well, honestly can't assess it properly now. Also
posted in http://www.reddit.com/r/lisp/comments/bsu83/type_calculation_from_the_ground_up/,
http://www.lispforum.com/viewtopic.php?f=29&t=696