docs: private void add(String pos, String s) {
  		if(s.length()>maxLen)
  			return;
  		if(pos.length()==0)
  			return;
  		posSet.add(pos);
  		if(dict.containsKey(s)){
  			TreeSet<String> sett = dict.get(s);
  			sett.add(pos);
  		}else{
  			TreeSet<String> sett  = new TreeSet<String>();
  			sett.add(pos);
  			dict.put(s, sett);
  		}
  	}