test.clj

(defn stateful-filter [pred event-key cache events]
  (lazy-seq
   (when (seq events)
     (let [event (first events)
           ekey (event-key event)]
       (if (pred (get cache ekey) event)
         (cons event (stateful-filter pred event-key
                                      (assoc cache ekey event) (rest events)))
         (stateful-filter pred event-key cache (rest events)))))))

(stateful-filter
 #(or (nil? %1)
      (nil? %2)
      (> (- (second %2) (second %1)) 10))
 first
 {}
 '[[a 1] [b 2] [a 3] [b 5] [a 11] [a 12] [b 13] [a 14] [b 16]])

;=> ([a 1] [b 2] [a 12] [b 13])