step5.cljs

(defn move-element [state from to element]
  (letfn [(remove* [elements ref]
            (into [] (remove #{ref} elements)))
          (add* [elements ref]
            (into [] (cond-> elements
                             (not (some #{ref} elements)) (conj ref))))]
    (-> state
        (update-in (conj from :elements) remove* element)
        (update-in (conj to :elements) add* element))))

(defmethod mutate 'element/drop
  [{:keys [state]} _ {:keys [from to element]}]
  {:value  {:keys [element from to]}
   :action (fn []
             (swap! state move-element from to element))})