;; Return the nth harmonic number
(define harm
  (lambda (n)
    (if (= n 1)
        1
        (+ (/ 1 n) (harm (- n 1))))))

;; Return m*n, whre m is a number
;; and n is a nonnegative integer
(define mult
  (lambda (m n)
    (if (= n 0)
        0
        (+ m (mult m (- n 1))))))

;; Return "n choose k" for nonnegative integer n
;; and integer k such that 0 <= k <= n
(define binom
  (lambda (n k)
    (if (or (= k 0) (= k n))
        1
        (+ (binom (- n 1) k)
           (binom (- n 1) (- k 1))))))

;; Count the number of 6's in the decimal representation
;; of nonnegative number n
(define number-of-6s
  (lambda (n)
    (if (= n 0)
        0
        (+ (if (= (remainder n 10) 6) 1 0)
           (number-of-6s (quotient n 10))))))

;; Russian Multiplication
;; Return m*n, whre m is a number
;; and n is a nonnegative integer
(define r-mult
  (lambda (m n)
    (if (= n 0)
        0
        (+ (r-mult (+ m m) (quotient n 2))
           (if (odd? n) m 0)))))

;; Return sum of digits in the decimal representation of a
;; nonnegative integer n
(define sum-of-digits
  (lambda (n)
    (if (= n 0)
        0
        (+ (sum-of-digits (quotient n 10))
           (remainder n 10)))))