Example keep-only implementation to keep at most n elements in each bucket of a sequence

gistfile1.clj

(defn keep-only
  [n bucket-fn s]
  (letfn [(my-filter
           [s seen]
           (when (seq s)
             (let [e (first s)
                   b (bucket-fn e)
                   c (inc (get seen b 0))]
               (if (> c n)
                 (recur (rest s) seen)
                 (cons e (lazy-seq (my-filter (rest s) (assoc seen b c))))))))]
    (my-filter s {})))