n C r

(define combination
  (λ(n k)
    (let ((stalled-factorial
           (λ(N stop-value)
             (define iter-sf
               (λ(accum N stop-value)
                 (if (= N stop-value)
                     accum
                     (iter-sf (* accum N) (- N 1) stop-value))))
             (iter-sf 1 N stop-value))))
      (if (> k (- n k))
          (/ (stalled-factorial n k) (factorial (- n k)))
          (/ (stalled-factorial n (- n k)) (factorial k))))))

Is it just me, or does LISP make everything 10x longer and harder to write? And you're telling me this is the language which the almighty programmers use? Please, don't make me laugh.