In addition, clojure.core has the handy tree-seq function:

    (defn tree-seq
      "Returns a lazy sequence of the nodes in a tree, via a depth-first walk.
       branch? must be a fn of one arg that returns true if passed a node
       that can have children (but may not).  children must be a fn of one
       arg that returns a sequence of the children. Will only be called on
       nodes for which branch? returns true. Root is the root node of the
      tree."
      {:added "1.0"
       :static true}
       [branch? children root]
       (let [walk (fn walk [node]
                    (lazy-seq
                     (cons node
                      (when (branch? node)
                        (mapcat walk (children node))))))]
         (walk root)))