Exercise 4.28.  Eval uses actual-value rather than eval to evaluate the 
operator before passing it to apply, in order to force the value of the 
operator. Give an example that demonstrates the need for this forcing. 

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(define (adder n) (lambda (x) (+ n x)))

(define add-7 (adder 7)) ;; add-7 is a thunk, adder has not yet been called

(add-7 12) ;; to apply add-7, it must be forced.