core.clj

(deftype Element [data start end] :as this
  clojure.lang.Associative
  (assoc [key value]
    (apply (fn [data start end]
             (Element. data start end))
           (vals (assoc {:data (:data this)
                         :start (:start this)
                         :end (:end this)}
                   key value)))))

;; much better:
(deftype Element [data start end]
  clojure.lang.IPersistentMap)

(assoc (Element "bla" 23 42) :data "blubb") => #:Element{:data "blubb", :start 23, :end 42}