Exercise 4.49.  Alyssa P. Hacker is more interested in generating 
interesting sentences than in parsing them. She reasons that by simply 
changing the procedure parse-word so that it ignores the ``input 
sentence'' and instead always succeeds and generates an appropriate 
word, we can use the programs we had built for parsing to do generation 
instead. Implement Alyssa's idea, and show the first half-dozen or so 
sentences generated.

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Implementation skipped.

As described in the footnote, these will not be very interesting, for 
the reason that the grammar is highly recursive, and can generate 
infinitely long sentences.

To fix this problem we can choose, before anything else, the length of 
the sentence we wish to generate, and then generate sentences of that 
length until no more can be found.  Then the limit will be increased and 
all the sentences with one more word will be generated, and so on.

This is an infinite stream, but as before with the infinite stream of 
integer triples, any given sentence, however long, will eventually be 
produced.

(define (generate-sentences)
  (let ((sentence-length (an-integer-starting-from 3))
        (words-generated 0))
    (define (parse-word words)
      (require (< words-generated sentence-length))
      (an-element-of words))
    (define (parse-sentence ...))
    ...
    the rest of the parsers should be redefined here, so as to use the new 'parse-word'
    ...
    (parse-sentence)))