(define rat 
  M N -> (let Vector (absvector 3) 
              TagVector (address-> Vector 0 rational-number) 
              Vector+M (address-> TagVector 1 M) 
              Vector+N (address-> Vector+M 2 N) 
              Vector+N)) 

(define rational-number 
     Vector -> (@s (str (<-address Vector 1)) 
                           "/"
                            (str (<-address Vector 2)))) 
                            
(rat 4 5)