; Return the plural form of a word
(define (plural wd)
  (if (equal? (last wd) 'y)
      (word (bl wd) 'ies)
      (word wd 's)))

; Return the value of a single roman numberal letter
(define (roman-value letter)
  (cond ((equal? letter 'M) 1000)
        ((equal? letter 'D) 500)
        ((equal? letter 'C) 100)
        ((equal? letter 'L) 50)
        ((equal? letter 'X) 10)
        ((equal? letter 'V) 5)
        ((equal? letter 'I) 1)
        (else (error "Invalid roman numeral " letter))))