To roman implementation in Clojure

Here is my take of to roman implementation in Clojure:
I posted this in http://rosettacode.org/wiki/Roman_Numerals#Clojure

1:  (def arabic-roman-map
2:     {1 "I", 5 "V", 
3:     10 "X", 50 "L", 
4:     100 "C", 500 "D", 
5:     1000 "M", 
6:     4 "IV", 9 "IX", 
7:     40 "XL", 90 "XC", 
8:     400 "CD", 900 "CM" })
9:  (def arabic-roman-map-sorted-keys
10:     (sort (keys arabic-roman-map)))
11:  (defn find-value-in-coll
12:   [coll k]
13:   (let [aval (find coll k)]
14:   (if (nil? aval) "" (val aval))))
15:  (defn to-roman
16:   [result n]
17:   (let
18:     [closest-key-for-n (last (filter #(> n %) arabic-roman-map-sorted-keys))
19:      roman-value-for-n (find-value-in-coll arabic-roman-map n)
20:      roman-value-for-closet-to-n (find-value-in-coll arabic-roman-map
21:                                   closest-key-for-n)]
22:      (if (or (<= n 0) (contains? arabic-roman-map n))
23:        (conj result roman-value-for-n)
24:        (recur (conj result roman-value-for-closet-to-n)
25:            (- n closest-key-for-n)))))
26:  Usage: >(to-roman [] 1999)
27:  result: ["M" "CM" "XC" "IX"]