Post-order traversal with a zipper in clojure

traversal.clj

;WARNING: this puts your entire tree in memory, so if you're using zippers
;to avoid that, do not use this.
;Also, potentially terrible clojure. I only learned this a few days ago
;couldn't find any way to do this online except for a single 
;stackoverflow that didn't actually use zippers
(require '[clojure.zip :as zip])
(defn zip-down [loc] 
  "Zips to the bottom of the tree from this loc"
  (loop [p loc]
    (if (zip/branch? p)
      (do (def cnext-stack (cons p cnext-stack))
          (recur (zip/down p)))
      p)))
;visited nodes
;remember to reset after a traversal
(def cnext-stack (list)) 

(defn cnext
  "Basically the same as clojure.zip/next, except postorder traversal."
  ([loc] (cnext loc cnext-stack))
  ([loc stack] 
   (if (= :end (loc 1))
     loc
     (or 
       ;branch node and visited before?
       (when (and (zip/branch? loc) (nil? (some #(= loc %) stack))) 
         (do (def cnext-stack (cons loc stack))
             (zip-down loc)))
       ;if sibling, go to it and zip down
       (when (zip/right loc) (-> loc zip/right zip-down)) 
       (if (zip/up loc)
         (zip/up loc)
         [(zip/node loc) :end])))))


(defn search 
  "Takes a zipper, and optionally a visit/edit function"
  ([zipper] (search zipper nil)) 
  ([zipper edit]  
   (do (def cnext-stack (list))
       (loop [loc zipper]
         (if (zip/end? loc)
           (zip/root loc)
           (do 
             (when edit (when-let [edited (edit loc)] 
                          (zip/replace loc edited))) 
             (recur (cnext loc cnext-stack))))))))