collapse.clj

(defn collapse
  "Collapses a tree of nested collections
  into a vector indexed by depth
  in the original structure."
  [tree]
  (loop [flat (transient [])
         tree tree]
    (if (empty? tree)
      (persistent! flat)
      (recur (conj! flat
                    (filterv (not (coll?))))
             (filterv coll? tree)))))