Exercise 4.25.  Suppose that (in ordinary applicative-order Scheme) we 
define unless as shown above and then define factorial in terms of 
unless  as

(define (factorial n)
  (unless (= n 1)
          (* n (factorial (- n 1)))
          1))

What happens if we attempt to evaluate (factorial 5)? Will our 
definitions work in a normal-order language?

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(factorial 5) will attempt to evaluate the arguments to unless before 
calling it, which will evaluate (factorial 4) and so on down to 
(factorial 1), and unfortunately on through 0, -1, -2, etc.  In a 
normal-order language, this will work, since when (factorial 1) is 
reached, the unless call will not use the first argument and so it will 
not be evaluated.