sicp-ex-3.77

  
  
  
 (define (integral delayed-integrand initial-value dt) 
         (cons-stream initial-value 
                 (let ((integrand (force delayed-integrand))) 
                         (if (stream-null? integrand) 
                                 the-empty-stream 
                                 (integral (delay (stream-cdr integrand)) 
                                           (+ (* dt (stream-car integrand)) 
                                                      initial-value) 
                                                dt)))))