sicp-ex-4.13

  
  
 ;; unbound variable in current frame 
 (define (unbound? expr) (tagged-list? expr 'unbound)) 
 (define (unbind-variable expr env) (make-unbound (cadr expr) env)) 
 (define (make-unbound variable env) 
         (let ((vars (frame-variables (first-frame env))) 
                   (vals (frame-values (first-frame env)))) 
                 (define (unbound vars vals new-vars new-vals) 
                         (cond ((null? vars) 
                                    (error "variable is not in the environment -- MAKE-UNBOUND"      
  
                                              variable)) 
                                  ((eq? (car vars) variable) 
                                   (set-car! env  
                                                 (cons (append new-vars (cdr vars))  
                                                          (append new-vals (cdr vals))))) 
                                  (else (unbound (cdr vars) (cdr vals)  
                                                           (cons (car vars) new-vars)  
                                                           (cons (car vals) new-vals))))) 
                 (unbound vars vals '() '()))) 
  
 ;; add this in eval 
 ((unbound? expr) (unbind-variable expr env))