(define (mul-series a b)
  (cons-stream (* (stream-car a) (stream-car b))
               (add-streams (scale-stream (stream-cdr a)
                                          (stream-car b))
                            (mul-series a (stream-cdr b)))))

(define (invert-unit-series s)
  (define x (cons-stream 1
                         (mul-series (scale-stream (stream-cdr s) -1)
                                     x)))
  x)