Exercise 5.41.  Write a procedure find-variable that takes as arguments 
a variable and a compile-time environment and returns the lexical 
address of the variable with respect to that environment. For example, 
in the program fragment that is shown above, the compile-time 
environment during the compilation of expression <e1> is ((y z) (a b c d 
e) (x y)). Find-variable should produce

(find-variable 'c '((y z) (a b c d e) (x y)))
(1 2)

(find-variable 'x '((y z) (a b c d e) (x y)))
(2 0)

(find-variable 'w '((y z) (a b c d e) (x y)))
not-found

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For reference, lookup-variable-value from ch5-eceval-support.scm:

(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)))))
  (env-loop env))

---

(define (find-variable var env)
  (define (env-loop env-index env)
    (define (scan frame-index vars vals)
      (cond ((null? vars)
             (env-loop (+ env-index 1) (enclosing-environment env)))
            ((eq? var (car vars))
             (make-lexical-address env-index frame-index))
            (else (scan (+ frame-index 1) (cdr vars) (cdr vals)))))
    (if (eq? env the-empty-environment)
        'not-found
        (let ((frame (first-frame env)))
          (scan 0
                (frame-variables frame)
                (frame-values frame)))))
  (env-loop 0 env))

(define make-lexical-address list)