Exercise 4.28: The actual-value call is necessary in the case where
the procedure to be applied is itself returned from another procudure.

Contrived example:

(define (choose-proc p1 p2) p1)
(define (f x) (+ x 1))
(define (g x) (+ x 4))

((choose-proc f g) 3)

Without the force-it call, we would have a thunk in the first position
of the expression, which would not work.