private void advance()
      {
         while(hasNext() && !st.hasMoreTokens())
            try
            {
               line = br.readLine();
               st = new StringTokenizer(line);
            }
               catch(Exception e)
               {
                  eof = true;
               }
         if(eof)
            token = null;
         else
            token = st.nextToken();
      }