RosettaCodeData/Task/Mutual-recursion/Scheme/mutual-recursion-2.ss

(letrec ((F (lambda (n)
              (if (= n 0) 1
                  (- n (M (F (- n 1)))))))
         (M (lambda (n)
              (if (= n 0) 0
                  (- n (F (M (- n 1))))))))
  (F 19)) # evaluates to 12