4Clojure problem 92 (Take One)

gistfile1.clj

(fn roman
  ([s] (roman (seq s) 0))
  ([s v]
    (let [c1 (first s)
          c2 (second s)
          c12 (list c1 c2)]
      (if (empty? s) v
        (cond
          (= c12 '(\I \V)) (roman (drop 2 s) (+ v 4))
          (= c12 '(\I \X)) (roman (drop 2 s) (+ v 9))
          (= c12 '(\X \L)) (roman (drop 2 s) (+ v 40))
          (= c12 '(\X \C)) (roman (drop 2 s) (+ v 90))
          (= c12 '(\C \D)) (roman (drop 2 s) (+ v 400))
          (= c12 '(\C \M)) (roman (drop 2 s) (+ v 900))
          (= c1 \M) (roman (rest s) (+ v 1000))
          (= c1 \D) (roman (rest s) (+ v 500))
          (= c1 \C) (roman (rest s) (+ v 100))
          (= c1 \L) (roman (rest s) (+ v 50))
          (= c1 \X) (roman (rest s) (+ v 10))
          (= c1 \V) (roman (rest s) (+ v 5))
          (= c1 \I) (roman (rest s) (+ v 1))
        )
      )
    )
  )
)