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Recursion on Integers and Lists

List processing procedures like sum are similar in form to those that work on integers, like sum-of-first. Compare: 
     (define sum
       (lambda (lst)
         (if (null? lst)
             0
             (+ (car lst) (sum (cdr lst))))))

     (define sum-of-first
       (lambda (n)
         (if (= n 0)
             0
             (+ n (sum-of-first (- n 1))))))
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