sicp-ex-3.76

  
  
 (define (smooth s) 
         (stream-map (lambda (x1 x2) (/ (+ x1 x2) 2)) 
                             (cons-stream 0 s) 
                                 s)) 
 (define (make-zeor-crosssings input-stream smooth) 
         (let ((after-smooth (smooth input-stream))) 
             (stream-map sign-change-detector after-smooth (cons-stream 0 after-smooth))))