1.41¶

;  Define a procedure `double` that takes a procedure of 
; one argument as argument and returns a procedure that 
; applies the original procedure twice. For example, if 
; `inc` is a procedure that adds 1 to its argument, then 
; (double inc) should be a procedure that adds 2. What 
; value is returned by
; (((double (double double)) inc) 5)

(define (double f) (lambda (x) (f (f x))))

(define (inc x) (+ x 1))
(((double (double double)) inc) 5) ;Value: 21