Combinators
Recursive Combinators
https://debasishg.blogspot.com/2008/12/what-is-special-about-combinators-in.html
http://www.kevinalbrecht.com/code/joy-mirror/j01tut.html
- Y combinator
linrec(condition, base case, recursive, final)
binrec()
tailrec(condition, base case, recursive)
primrec(recursive, final)