Exercise 4.70.  What is the purpose of the let bindings in the 
procedures add-assertion! and add-rule! ? What would be wrong with the 
following implementation of add-assertion! ? Hint: Recall the definition 
of the infinite stream of ones in section 3.5.2: (define ones 
(cons-stream 1 ones)).

(define (add-assertion! assertion)
  (store-assertion-in-index assertion)
  (set! THE-ASSERTIONS
        (cons-stream assertion THE-ASSERTIONS))
  'ok)

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The second argument to cons-stream is deferred.

In the case of a list:

(define l '(b c))
(set! l (cons 'a l))

When the cons is evaluated, l will be looked up, and the current value 
used in the list cons returns.  However, in the case of cons-stream as 
above, the reference to THE-ASSERTIONS will be evaluated only when it 
is needed, at which point the variable name THE-ASSERTIONS has already 
been assigned the newly extended stream by the set! call.