I don't know if I would call it fun, but SICP made me do it. I will say though, recursive function expansion can make some pretty trees.

;; The Ackermannn function in all of it's glory. (Scheme) ;; https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book-Z-H-11.html (define (A x y) (cond ((= y 0) 0) ((= x 0) (* 2 y)) ((= y 1) 2) (else (A (- x 1) (A x (- y 1))))))

;; h as an Ackermann variant (define (h n) (A 2 n))

;; expansion 1 (= (h 1) (A 2 1) 2)

;; expansion 2 (= (h 2) (A 2 2) (A 1 (A 2 1)) (A 1 2) (A 0 (A 1 1)) (A 0 2) 4)

;; expansion 3 (= (h 3) (A 2 3) (A 1 (A 2 2)) (A 1 (A 0 (A 2 1))) (A 1 (A 0 2)) (A 1 4) (A 0 (A 1 3)) (A 0 (A 0 (A 1 2))) (A 0 (A 0 (A 0 (A 1 1)))) (A 0 (A 0 (A 0 2))) (A 0 (A 0 4)) (A 0 8) 16))

;; expansion 4 (= (h 4) (A 2 4) (A 1 (A 2 3)) (A 1 (A 1 (A 2 2))) (A 1 (A 1 (A 1 (A 2 1)))) (A 1 (A 1 (A 1 2))) (A 1 (A 1 (A 0 (A 1 1)))) (A 1 (A 1 (A 0 2))) (A 1 (A 1 4)) (A 1 (A 0 (A 1 3))) (A 1 (A 0 (A 0 (A 1 2)))) (A 1 (A 0 (A 0 (A 0 (A 1 1))))) (A 1 (A 0 (A 0 (A 0 2)))) (A 1 (A 0 (A 0 4))) (A 1 (A 0 8)) (A 1 16) (A 0 (A 1 15)) (A 0 (A 0 (A 1 14))) (A 0 (A 0 (A 0 (A 1 13)))) (A 0 (A 0 (A 0 (A 0 (A 1 12))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 11)))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 10))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 9)))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 8))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 7)))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 6))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 5)))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 4))))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 3)))))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 2))))))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 1)))))))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 2))))))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 4)))))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 8))))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 16)))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 32))))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 64)))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 128))))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 256)))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 512))))))) (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 1024)))))) (A 0 (A 0 (A 0 (A 0 (A 0 2048))))) (A 0 (A 0 (A 0 (A 0 4096)))) (A 0 (A 0 (A 0 8192))) (A 0 (A 0 16384)) (A 0 32768) 65536)