(define sum-of-divisors
  (lambda (n)
    (define sum-from-plus ; sum of all divisors of n which are
      (lambda (low addend) ; >= low, plus addend
        (if (> low n)
            addend    ; no divisors of n are greater than n
            (sum-from-plus (+ low 1) 
                           (if (divides? low n)
                               (+ addend low)
                               addend)))))
    (sum-from-plus 1 0)))

(define divides?
  (lambda (a b)
    (= (remainder b a) 0)))