(defn flatten-nodes-coll [x]
  (letfn [(flat* [x stack]
            (if (node? x) 
              (cons x (when (seq stack) (flat (peek stack) (pop stack))))
              (if-let [[x & xs] (seq x)]
                (recur x (conj stack xs))
                (when (seq stack)
                  (recur (peek stack) (pop stack))))))
          (flat [x stack]
            (lazy-seq (flat* x stack)))]
    (flat x ())))