code: public int getTileSetID (String tileSetName)
          throws PersistenceException
      {
          Integer tsid = _map.get(tileSetName);
          if (tsid == null) {
              tsid = Integer.valueOf(++_nextTileSetID);
              _map.put(tileSetName, tsid);
          }
          return tsid.intValue();
      }
docs: documentation inherited