Clojure index-of

(defn- seq-index-of
  "If ys is a subsquence of xs, return the index of where it occurs, nil
  otherwise."
  [xs ys]
  (let [slv? (some-fn seq? list? vector?)]
   (when (and (slv? xs) (slv? ys))
     (let [idx (.indexOf xs (first ys))]
       (when (and ((complement neg?) idx)
                  (= ys (take (count ys) (drop idx xs))))
         idx)))))