#lang racket

(define (mul-series s1 s2)
  (cons-stream (* (stream-car s1)
                  (stream-car s2))
               (add-streams (scale-stream (stream-cdr s1) (stream-car s2))
                            (add-streams 
                             (scale-stream (stream-cdr s2) (stream-car s1))
                             (mul-series (stream-cdr s1) 
                                         (stream-cdr s2))))))