Add roman numerals in Common Lisp

roman-calc.lisp

(defun mapcn (chars nums string)
  (loop as char across string
        as i = (position char chars)
        collect (and i (nth i nums))))
 
(defun parse-roman (R)
  (loop with nums = (mapcn "IVXLCDM" '(1 5 10 50 100 500 1000) R)
        as (A B) on nums if A sum (if (and B (< A B)) (- A) A)))
  
(defun +roman (&rest rx)
  (format nil "~@R"
    (reduce #'+ (mapcar #'parse-roman rx))))
  
;;
  
(+roman "I" "III" "M") ;==> "MIV"