sicp-ex-4.52

  
  
 ;; add this in analyze 
 ((if-fail? expr) (analyze-if-fail expr)) 
  
 ;; add those to amb evaluator 
 (define (if-fail? expr) (tagged-list? expr 'if-fail)) 
  
  (define (analyze-if-fail expr) 
   (let ((first (analyze (cadr expr))) 
         (second (analyze (caddr expr)))) 
    (lambda (env succeed fail) 
     (first env 
            (lambda (value fail2) 
             (succeed value fail2)) 
            (lambda () 
             (second env succeed fail))))))