Exercise 3.61.  Let S be a power series (exercise 3.59) whose constant 
term is 1. Suppose we want to find the power series 1/S, that is, the 
series X such that S · X = 1. Write S = 1 + SR  where SR is the part of 
S after the constant term. Then we can solve for X as follows:

In other words, X is the power series whose constant term is 1 and whose 
higher-order terms are given by the negative of SR times X. Use this 
idea to write a procedure invert-unit-series that computes 1/S for a 
power series S with constant term 1. You will need to use mul-series 
from exercise 3.60.

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