Exercise 1.38.  In 1737, the Swiss mathematician Leonhard Euler published a 
memoir De Fractionibus Continuis, which included a continued fraction expansion 
for e - 2, where e is the base of the natural logarithms. In this fraction, the 
Nᵢ are all 1, and the Dᵢ are successively 1, 2, 1, 1, 4, 1, 1, 6, 1, 1, 8, .... 
Write a program that uses your cont-frac procedure from exercise 1.37 to 
approximate e, based on Euler's expansion. 

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(define (euler-e k)
  (+ 2 (cont-frac (lambda (n) 1.0)
                  (lambda (d) (if (= (remainder d 3) 2)
                                  (* 2/3 (+ d 1))
                                  1))
                  k)))