(define (count-set-bits n)
  (let loop ((n n)
             (set-bits 0))
    (if (zero? n)
        set-bits
        (loop (bitwise-and n (fx- n 1))
              (fx+ set-bits 1)))))

(let loop ((n 0))
  (when (< n 16)
    (print "n=" n "  set bits=" (count-set-bits n))
    (loop (+ n 1))))