(doseq [{id :_id tags :tags diff :difficulty} (fetch :problems)]
  (when-not diff
    (let [[[diff] tags] ((juxt filter remove) #{"easy" "elementary" "medium" "hard"} tags)]
      (update! :problems {:_id id}
               {:$set {:difficulty (clojure.contrib.string/capitalize (or diff "")), :tags tags}}
               :upsert false, :multiple true))))