4Clojure problem 92 (Take Two)

gistfile1.clj

(fn roman
  ([s] (roman (seq s) 0))
  ([s v]
    (let [
          rdigits {\M 1000, \D 500, \C 100, \L 50, \X 10, \V 5, \I 1}
          sdigits {"IV" 4, "IX" 9, "XL" 40, "XC" 90, "CD" 400, "CM" 900}
          c1  (first s)
          c12 (str c1 (second s))
          v1  (get rdigits c1 0)
          v12 (get sdigits c12)]
      (if (empty? s) v
        (if (nil? v12)
          (roman (rest s) (+ v v1))
          (roman (drop 2 s) (+ v v12))
        )
      )
    )
  )
)