sicp-ex-5.44

  
  
  
 (define (overwrite? operator ct-env) 
  (let ((r (find-variable operator ct-env))) 
   (eq? r 'not-found))) 
 (define (open-code? exp ct-env) 
  (and (memq (car exp) '(+ - * /)) 
          (overwrite? (car exp) ct-env)))