http://stackoverflow.com/a/31246182/1202461

so-31242478.clj

(defn flatten-bfs [{:keys [attrs content]}]
  (loop [ret []
         queue (into clojure.lang.PersistentQueue/EMPTY content)]
    (if (seq queue)
      (let [{attrs :attrs children :content} (peek queue)]
        (recur (if (nil? (:v attrs))
                   ret
                   (conj ret (:v attrs)))
               (into (pop queue) children)))
      ret)))