substring Roman->Number

;; Constants
(define MarcusAurelius (cons "I" (cons "V" (cons "X" (cons "L"
                       (cons "C" (cons "D" (cons "M" empty))))))))

(define panem_et_circenses (cons 1 (cons 5 (cons 10 (cons 50
                           (cons 100 (cons 500 (cons 1000 empty))))))))

(define (pos? input stoic counter) ;my way of thinking about recursion isn't strong, taking the easy way out
  (cond                            ;w/ a counter parameter. Is it really possible to do this with recursion?!
      [(= (string-length input) 0) 0]
      [(string=? (substring input 0 1) (first stoic)) counter]
      [else (pos? input (rest stoic) (add1 counter))]))

(define (list-ref?! Arabic pos)
  (cond
    [(empty? Arabic) false]
    [(= pos 0) (first Arabic)]
    [else (list-ref?! (rest Arabic) (- pos 1))]))

(define (roman->number rnum)
  (cond
    [(= (string-length rnum) 0) 0]
    [(= (string-length rnum) 1) (list-ref?! panem_et_circenses (pos? rnum MarcusAurelius 0))]
    [else
     (cond
       [(and (< (pos? (substring rnum 0 1) MarcusAurelius 0) (pos? (substring rnum 1 2) MarcusAurelius 0)) ;IV IX XL XC CD CM
             (or (and (= 0 (pos? (substring rnum 0 1) MarcusAurelius 0))
                      (or (= 1 (pos? (substring rnum 1 2) MarcusAurelius 0)) (= 2 (pos? (substring rnum 1 2) MarcusAurelius 0))))
                 (and (= 2 (pos? (substring rnum 0 1) MarcusAurelius 0))
                      (or (= 3 (pos? (substring rnum 1 2) MarcusAurelius 0)) (= 4 (pos? (substring rnum 1 2) MarcusAurelius 0))))
                 (and (= 4 (pos? (substring rnum 0 1) MarcusAurelius 0))
                      (or (= 5 (pos? (substring rnum 1 2) MarcusAurelius 0)) (= 6 (pos? (substring rnum 1 2) MarcusAurelius 0))))))
        (+ (- (list-ref?! panem_et_circenses (pos? (substring rnum 1 2) MarcusAurelius 0))
              (list-ref?! panem_et_circenses (pos? (substring rnum 0 1) MarcusAurelius 0)))
        (roman->number (substring rnum 2)))]
       [else (+ (list-ref?! panem_et_circenses (pos? (substring rnum 0 1) MarcusAurelius 0)) (roman->number (substring rnum 1)))])]))