/**
  * last strip id fetched in this comic collection so we only retrieve the new one
  *
  * @param comic
  * @return last fetched strip id or if no strip at all, the string "unknown"
  */
 @NonNull
 private String getLastId(String comic) {
   String lastId = cache.lastStripId(comic);
   if (lastId == null) {
     lastId = "unknown";
   }
   return lastId;
 }
 @Override
 public Observable<List<Strip>> model(String comic) {
   return cache.listStrips(comic);
 }