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Add roman numerals in Common 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"
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