sicp-ex-5.41

  
  
 (define (find-variable variable lst) 
   (define (search-variable v l n) 
     (cond ((null? l) false) 
           ((eq? v (car l)) n) 
           (else (search-variable v (cdr l) (+ n 1))))) 
   (define (search-frame frames f) 
     (if (null? frames) 
         'not-found 
         (let ((o (search-variable variable (car frames) 0))) 
              (if o 
                  (cons f o) 
                  (search-frame (cdr frames) (+ f 1)))))) 
   (search-frame lst 0))