lazy-stuff.clj

(defn tree->seq [tree]
  (letfn [(tree->seq-aux
           [stack]
           (if-let [s (seq stack)]
             (let [{:keys [value left right]} (first stack)]
               (lazy-seq
                (cons value
                      (tree->seq-aux
                       (concat (keep identity [left right])
                               (rest stack))))))
             '()))]
    (tree->seq-aux [tree])))