SICP in Clojure: Chapter 1, Exercise 41

 This is my Clojure solution to Chapter 1, Exercise 41:

(ns sicp.ch1.ex41)

(defn double-it
  [f]
  #(f (f %))) ; could use comp

(comment
  (((double-it (double-it double-it)) inc) 5) ; => 21
  #__)