docs: public int lookupIndex(String str, int indent) {
  		if (indent < 1)
  			throw new IllegalArgumentException(
  					"Invalid Argument in FeatureAlphabet: " + indent);
  
  		int ret = data.get(str);
  
  		if (ret==-1 && !frozen) {//字典中没有，并且允许插入
  			synchronized (this) {
  				data.put(str, last);
  				ret = last;
  				last += indent;
  			}
  		}
  //		if(ret==0)
  //			System.out.println(str);
  		return ret;
  	}