;; A list containing the values of single-letter Roman numerals.
(setf *roman-number-list*
  '((I 1) (V 5) (X 10) (L 50) (C 100) (D 500) (M 1000)))

;; Converts a single Roman numeral letter into its equivalent decimal value.
(defun letter-to-decimal (letter)
  (car (cdr ((lambda (lst f)
               (cond ((null lst) ())
                     ((eq (car (car lst)) letter) (car lst))
                     (t (funcall f (cdr lst) f))))
             *roman-number-list*
             (lambda (lst f)
               (cond ((null lst) ())
                     ((eq (car (car lst)) letter) (car lst))
                     (t (funcall f (cdr lst) f))))))))