Knuth's up-arrow notation

(defun knuth (x n)
  (if (= n 2)
      (return-from knuth (expt x x))
    (return-from knuth (expt x (knuth x (- n 1))))
    )
)


Yes, an implementation in 6 lines of Lisp. What is Knuth's up-arrow notation? Wikipedia that shit. Don't try this with anything greater than (knuth 3 3), or you will overflow. (knut 3 3) == 3^3^3. (knuth 3 4) == 3^3^3^3.

>>66
You seem to be a so-called ``EXPERT BBCODE PROGRAMMER''. In the wake of BBCode 3.0, I'm writing a Haskell library that enables mere mortals to write ``proper'' BBCode. What particular BBCode features do you find useful? What kind of functionality do you except from an ENTERPRISE-quality BBCode solution?