Exercise 4.47: Louis Reasoner suggests that,
since a verb phrase is either a verb or a verb phrase followed by a
prepositional phrase, it would be much more straightforward to define the
procedure parse-verb-phrase as follows (and similarly for noun phrases):


(define (parse-verb-phrase)
  (amb (parse-word verbs)
       (list 
        'verb-phrase
        (parse-verb-phrase)
        (parse-prepositional-phrase))))

Does this work?  Does the program’s behavior change if we interchange the order
of expressions in the amb?