(define (fact-tail-rec n r)
  (if (= 0 n)
      r
      (fact-tail-rec (- n 1) (* r n))))


(define (fact-tail-rec-cps-1 n r k)
  (if (= 0 n)
      (k r)
      (fact-tail-rec-cps-1
        (- n 1)
        (* r n)
        (lambda (v)  ; Eventually v becomes (fact n), because the base case passes 
           (k v))    ; the result via a chain of trivial "pass on" functions.
                     ; Are all these (lambda(v) (k v)) functions really necessary?
      )              ; No - see the next variant called fact-tail-rec-cps-2.
  )
)


(define (fact-tail-rec-cps-2 n r k)
  (if (= 0 n)
      (k r)
      (fact-tail-rec-cps-2
        (- n 1)
        (* r n)
        k           ; Eventually (fact n) is passed to k. k is the continuation 
      )             ; of the original call to the factorial function.
   )
)