cljs.repl/merge-spec

(defn merge-spec [[lib & {:as aindex}] [_ & {:as bindex}]]
  (let [merged-map
        (merge-with
          (fn [x y]
            (if (vector? x)
              (vec (distinct (into x y)))
              y))
          aindex bindex)]
    (apply vector lib
      (apply concat
        (sort
          (fn [[sa] [sb]]
            (compare (spec-sort sa) (spec-sort sb)))
          merged-map)))))