sicp-ex-4.20

  
  
  
 ;; a 
 ;; letrec expression 
 (define (letrec? expr) (tagged-list? expr 'letrec)) 
 (define (letrec-inits expr) (cadr expr)) 
 (define (letrec-body expr) (cddr expr)) 
 (define (declare-variables expr) 
         (map (lambda (x) (list (car x) '*unassigned*)) (letrec-inits expr))) 
 (define (set-variables expr) 
         (map (lambda (x) (list 'set! (car x) (cadr x))) (letrec-inits expr))) 
 (define (letrec->let expr) 
         (list 'let (declare-variables expr)  
                (make-begin (append (set-variables expr) (letrec-body expr)))))