Clojure: parse sequential opts delimited by a known set of names

opts.clj

(defn parse-opts [ks aseq]
  (let [ks (set ks)]
    (loop [last-k nil
           opts {}
           frontier aseq]
      (if-let [value (first frontier)]
        (if (ks value)
            (recur value opts (rest frontier))
            (recur last-k (update-in opts [last-k] (fnil conj []) value) (rest frontier)))
        opts))))