private static int getAttributeId(String attId) throws IOException {
      if (myAttributeIds.containsKey(attId)) {
        return myAttributeIds.get(attId);
      }

      int id = myNames.enumerate(attId);
      myAttributeIds.put(attId, id);

      return id;
    }
 private static void persist(Set<String> groups, PersistentStringEnumerator persistent)
     throws IOException {
   for (String each : groups) {
     persistent.enumerate(each);
   }
 }