clojure exercise

gistfile1.clj

(def tree [:f [:b [:a nil nil] [:d [:c nil nil] [:e nil nil]]] [:g nil [:i [:h nil nil] nil]]])

(fact
  (depth-first-preorder tree) => [:f :b :a :d :c :e :g :i :h])

;cheating? =)
(defn depth-first-preorder
  "Perform a preorder traversal of a tree, depth-first."
  [tree]
  (filter #(not (nil? %)) (flatten tree)))