Exercise 4.16.  In this exercise we implement the method just described 
for interpreting internal definitions. We assume that the evaluator 
supports let (see exercise 4.6).

a.  Change lookup-variable-value (section 4.1.3) to signal an error if 
the value it finds is the symbol *unassigned*.

b.  Write a procedure scan-out-defines that takes a procedure body and 
returns an equivalent one that has no internal definitions, by making 
the transformation described above.

c.  Install scan-out-defines in the interpreter, either in 
make-procedure or in procedure-body (see section 4.1.3). Which place is 
better? Why?

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a)

(define (lookup-variable-value var env)
  (define (env-loop env)
    (define (scan vars vals)
      (cond ((null? vars)
             (env-loop (enclosing-environment env)))
            ((eq? var (car vars))
             (car vals))
            (else (scan (cdr vars) (cdr vals)))))
    (if (eq? env the-empty-environment)
        (error "Unbound variable" var)
        (let ((frame (first-frame env)))
          (scan (frame-variables frame)
                (frame-values frame)))))
  (if (eq? var '*unassigned*)
      (error "reference to a variable not yet initialized")
      (env-loop env)))

b)

skipped

c)

Make-procedure is better, since a procedure will only be made once but 
may be used many times.