Exercise 4.46.  The evaluators in sections 4.1 and 4.2 do not determine 
what order operands are evaluated in. We will see that the amb evaluator 
evaluates them from left to right. Explain why our parsing program 
wouldn't work if the operands were evaluated in some other order.

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Consider parse-simple-noun-phrase:

(define (parse-simple-noun-phrase)
  (list 'simple-noun-phrase
        (parse-word articles)
        (parse-word nouns)))

The unparsed input is stored in a mutable global variable, so if 
(parse-word nouns) was evaluated before (parse-word articles), the input 
tokens would be consumed out of order and the parse would fail or give 
incorrect results.