test4.clj

(defn numseq []
  (->> (range) (map inc) (map #(.toString %)) (map seq) (flatten)))

(defn positions-of [el s]
  (let [el' (-> el .toString seq)]
    (->> s
         (partition (count el') 1)
         (map-indexed vector)
         (filter (fn [[i x]] (= x el')))
         (map (comp inc first)))))

(defn find-first [el s]
  (first (positions-of el s)))