Deep merge with vector, seq and set handling

deep_merge.clj

(defn- deep-merge
  "Takes a collection of maps and deep merges them. I.e...

   [{:a {:b 1}}
    {:a {:c 2}}]

  ... will be merged into ...

  [{:a {:b 1
        :c 2}}]

  If the maps contain vectors for the same path into the map, those
  vectors are concatenated. The behaviour for when the maps contain
  vectors for the same path where other maps contain other values,
  is undefined."
  [& maps]
  (let [f (fn [old new]
            (condp every? [old new]
              map? (deep-merge old new)
              vector? (into old new)
              seq? (concat old new)
              set? (into old new)
              new))]
    (apply merge-with f maps)))